It doesn't look as if it is possible to change the start and finish dates of any financial year in Acquisitions Configuration > Fund And Ledger Fiscal Period table mapping tables once the year has been created, unless it is deleted completely.

Can it be made possible for the start and (in particular) the end dates to be edited having already been set and saved, if so required?

Being able to do this would be of particular benefit around the time of a financial year rollover, especially if an institution cannot be certain that all financial transactions for the outgoing year will be completed in time for the originally set end date (necessitating, therefore, an extension of the outgoing financial year and a delaying of the incoming one).