diff --git a/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.cpp b/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.cpp
index 6e83dd4fdc..e3c0e3e2ca 100644
--- a/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.cpp
+++ b/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.cpp
@@ -42,18 +42,9 @@ std::expected<RimCase*, QString> RicfForwarding::findCase( int caseId )
     RimProject* project = RimProject::current();
     if ( !project ) return std::unexpected( "No project is available." );
 
-    std::vector<RimCase*> allCases = project->allGridCases();
-    if ( caseId < 0 )
-    {
-        // Matches RiaProjectModifier::firstCaseId(): the first occurrence is the first Eclipse result case
-        for ( RimCase* rimCase : allCases )
-        {
-            if ( dynamic_cast<RimEclipseResultCase*>( rimCase ) ) return rimCase;
-        }
-        return std::unexpected( "No Eclipse result cases found in project." );
-    }
+    if ( caseId < 0 ) return std::unexpected( "No case id specified." );
 
-    for ( RimCase* rimCase : allCases )
+    for ( RimCase* rimCase : project->allGridCases() )
     {
         if ( rimCase && rimCase->caseId() == caseId ) return rimCase;
     }
@@ -61,6 +52,24 @@ std::expected<RimCase*, QString> RicfForwarding::findCase( int caseId )
     return std::unexpected( QString( "Could not find case with ID %1" ).arg( caseId ) );
 }
 
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+std::expected<RimCase*, QString> RicfForwarding::findCaseOrFirstEclipseResultCase( int caseId )
+{
+    if ( caseId >= 0 ) return findCase( caseId );
+
+    RimProject* project = RimProject::current();
+    if ( !project ) return std::unexpected( "No project is available." );
+
+    // Matches RiaProjectModifier::firstCaseId(): the first occurrence is the first Eclipse result case
+    for ( RimCase* rimCase : project->allGridCases() )
+    {
+        if ( dynamic_cast<RimEclipseResultCase*>( rimCase ) ) return rimCase;
+    }
+    return std::unexpected( "No Eclipse result cases found in project." );
+}
+
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
@@ -95,11 +104,10 @@ std::expected<Rim3dView*, QString> RicfForwarding::findView( int viewId )
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
-std::expected<RimIdenticalGridCaseGroup*, QString> RicfForwarding::findCaseGroup( int groupId )
+namespace
+{
+std::vector<RimIdenticalGridCaseGroup*> allCaseGroups( RimProject* project )
 {
-    RimProject* project = RimProject::current();
-    if ( !project ) return std::unexpected( "No project is available." );
-
     std::vector<RimIdenticalGridCaseGroup*> caseGroups;
     for ( RimOilField* oilField : project->oilFields() )
     {
@@ -110,14 +118,21 @@ std::expected<RimIdenticalGridCaseGroup*, QString> RicfForwarding::findCaseGroup
             if ( caseGroup ) caseGroups.push_back( caseGroup );
         }
     }
+    return caseGroups;
+}
+} // namespace
 
-    if ( groupId < 0 )
-    {
-        if ( caseGroups.empty() ) return std::unexpected( "No grid case groups found in project." );
-        return caseGroups.front();
-    }
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+std::expected<RimIdenticalGridCaseGroup*, QString> RicfForwarding::findCaseGroup( int groupId )
+{
+    RimProject* project = RimProject::current();
+    if ( !project ) return std::unexpected( "No project is available." );
 
-    for ( RimIdenticalGridCaseGroup* caseGroup : caseGroups )
+    if ( groupId < 0 ) return std::unexpected( "No grid case group id specified." );
+
+    for ( RimIdenticalGridCaseGroup* caseGroup : allCaseGroups( project ) )
     {
         if ( caseGroup->groupId() == groupId ) return caseGroup;
     }
@@ -125,6 +140,21 @@ std::expected<RimIdenticalGridCaseGroup*, QString> RicfForwarding::findCaseGroup
     return std::unexpected( QString( "Could not find grid case group with ID %1" ).arg( groupId ) );
 }
 
+//--------------------------------------------------------------------------------------------------
+///
+//--------------------------------------------------------------------------------------------------
+std::expected<RimIdenticalGridCaseGroup*, QString> RicfForwarding::findCaseGroupOrFirst( int groupId )
+{
+    if ( groupId >= 0 ) return findCaseGroup( groupId );
+
+    RimProject* project = RimProject::current();
+    if ( !project ) return std::unexpected( "No project is available." );
+
+    auto caseGroups = allCaseGroups( project );
+    if ( caseGroups.empty() ) return std::unexpected( "No grid case groups found in project." );
+    return caseGroups.front();
+}
+
 //--------------------------------------------------------------------------------------------------
 ///
 //--------------------------------------------------------------------------------------------------
diff --git a/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.h b/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.h
index 97f08160f3..9c62bd509a 100644
--- a/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.h
+++ b/ApplicationLibCode/CommandFileInterface/RicfCommandForwarding.h
@@ -43,18 +43,26 @@ class PdmObjectHandle;
 //==================================================================================================
 namespace RicfForwarding
 {
-/// Find a grid case by id. A negative id resolves to the first Eclipse result case in the project.
+/// Find a grid case by id. A negative id is not accepted.
 std::expected<RimCase*, QString> findCase( int caseId );
 
+/// Find a grid case by id, or the first Eclipse result case in the project when the id is negative.
+/// Only for legacy commands that documented this fallback (replaceCase).
+std::expected<RimCase*, QString> findCaseOrFirstEclipseResultCase( int caseId );
+
 /// Find a view by id in the given case. A negative id is not accepted.
 std::expected<Rim3dView*, QString> findView( RimCase* rimCase, int viewId );
 
 /// Find a 3D view by id anywhere in the project. A negative id is not accepted.
 std::expected<Rim3dView*, QString> findView( int viewId );
 
-/// Find a grid case group by id. A negative id resolves to the first case group in the project.
+/// Find a grid case group by id. A negative id is not accepted.
 std::expected<RimIdenticalGridCaseGroup*, QString> findCaseGroup( int groupId );
 
+/// Find a grid case group by id, or the first case group in the project when the id is negative.
+/// Only for legacy commands that documented this fallback (replaceSourceCases).
+std::expected<RimIdenticalGridCaseGroup*, QString> findCaseGroupOrFirst( int groupId );
+
 /// Find a statistics case by case id in any grid case group.
 std::expected<RimEclipseStatisticsCase*, QString> findStatisticsCase( int caseId );
 
diff --git a/ApplicationLibCode/CommandFileInterface/RicfReplaceCase.cpp b/ApplicationLibCode/CommandFileInterface/RicfReplaceCase.cpp
index 60370ac261..b16feb5874 100644
--- a/ApplicationLibCode/CommandFileInterface/RicfReplaceCase.cpp
+++ b/ApplicationLibCode/CommandFileInterface/RicfReplaceCase.cpp
@@ -81,7 +81,7 @@ caf::PdmScriptResponse RicfSingleCaseReplace::execute()
         projectPath = lastProjectPath;
     }
 
-    auto rimCase = RicfForwarding::findCase( m_caseId() );
+    auto rimCase = RicfForwarding::findCaseOrFirstEclipseResultCase( m_caseId() );
     if ( !rimCase ) return RicfForwarding::errorResponse( rimCase.error(), commandName );
 
     RimCase_replaceGrid method( rimCase.value() );
diff --git a/ApplicationLibCode/CommandFileInterface/RicfReplaceSourceCases.cpp b/ApplicationLibCode/CommandFileInterface/RicfReplaceSourceCases.cpp
index dc0e4a0946..6b95dcade3 100644
--- a/ApplicationLibCode/CommandFileInterface/RicfReplaceSourceCases.cpp
+++ b/ApplicationLibCode/CommandFileInterface/RicfReplaceSourceCases.cpp
@@ -57,7 +57,7 @@ caf::PdmScriptResponse RicfReplaceSourceCases::execute()
                                               commandName );
     }
 
-    auto caseGroup = RicfForwarding::findCaseGroup( m_caseGroupId() );
+    auto caseGroup = RicfForwarding::findCaseGroupOrFirst( m_caseGroupId() );
     if ( !caseGroup ) return RicfForwarding::errorResponse( caseGroup.error(), commandName );
 
     RimIdenticalGridCaseGroup_replaceSourceCases method( caseGroup.value() );