Permit changing my own username's case without an error saying it is already taken

app/controllers/users_controller.rb

@@ -96,6 +96,9 @@ class UsersController < ApplicationController
   def check_username
     params.require(:username)
 
+    # The special case where someone is changing the case of their own username
+    return render(json: {available: true}) if current_user and params[:username].downcase == current_user.username.downcase
+
     validator = UsernameValidator.new(params[:username])
     if !validator.valid_format?
       render json: {errors: validator.errors}

app/models/user.rb

@@ -159,9 +159,10 @@ class User < ActiveRecord::Base
   end
 
   def change_username(new_username)
-    current_username, self.username = username, new_username
+    current_username = self.username
+    self.username = new_username
 
-    if SiteSetting.call_discourse_hub? && valid?
+    if current_username.downcase != new_username.downcase && SiteSetting.call_discourse_hub? && valid?
       begin
         DiscourseHub.change_nickname(current_username, new_username)
       rescue DiscourseHub::NicknameUnavailable
@@ -566,7 +567,8 @@ class User < ActiveRecord::Base
   def username_validator
     username_format_validator || begin
       lower = username.downcase
-      if username_changed? && User.where(username_lower: lower).exists?
+      existing = User.where(username_lower: lower).first
+      if username_changed? && existing && existing.id != self.id
         errors.add(:username, I18n.t(:'user.username.unique'))
       end
     end

spec/controllers/users_controller_spec.rb

@@ -772,6 +772,26 @@ describe UsersController do
         json['errors'][0].should_not be_nil
       end
     end
+
+    describe 'different case of existing username' do
+      context "it's my username" do
+        let!(:user) { Fabricate(:user, username: 'hansolo') }
+        before do
+          log_in_user(user)
+          xhr :get, :check_username, username: 'HanSolo'
+        end
+        include_examples 'when username is available everywhere'
+      end
+
+      context "it's someone else's username" do
+        let!(:user) { Fabricate(:user, username: 'hansolo') }
+        before do
+          log_in
+          xhr :get, :check_username, username: 'HanSolo'
+        end
+        include_examples 'when username is unavailable locally'
+      end
+    end
   end
 
   describe '.invited' do

spec/models/user_spec.rb

@@ -196,6 +196,19 @@ describe User do
       end
     end
 
+    describe 'change the case of my username' do
+      let!(:myself) { Fabricate(:user, username: 'hansolo') }
+
+      it 'should return true' do
+        myself.change_username('HanSolo').should be_true
+      end
+
+      it 'should change the username' do
+        myself.change_username('HanSolo')
+        myself.reload.username.should == 'HanSolo'
+      end
+    end
+
   end
 
   describe 'delete posts' do