The following changes occur in the Gnucash GUI when the "Euro support"
preference is enabled:

1) In any register denominated in a national currency that been
   replaced by the Euro (e.g. an account in DEM), the summarybar will
   show totals in both the old currency and in EUR.

2) There is an additional line labeled "EUR, total" in the account
   tree summarybar.  The totals in this line are a sum of all accounts
   denominated in currencies that have been replaced by the Euro.
   (E.G. The sum of all accounts in DEM, plus all accounts in FRF,
   etc., etc.)