Yes. you can however existing encumbrances are not cleared from the feeder systems. Therefore it is not recommended. If you do change the budgetary control options for an existing set of books, you must do two things for the change to be reflected.
--Run the Period Map Maintenance concurrent request, it must complete successfully.
--Exit Oracle Applications and restart.
You must completely exit the application...it is not sufficient to select Sign on again from the Oracle Applications Special menu. Ref: Oracle GL User's Guide, Rel 11, Vol 1, page 2-90