I did the same in 2011 for an Interbase 2007 database with Delphi XE and IBDac components.
We have be working with the new UTF-8 database since december 2011 and I have not seen any issues with the database migration from NONE to UTF-8.
I will not guarantee 100%, but it looks like the conversion went flawless.
Because all Datasets (FIBPlus or IBDac) or inherited from the original TDataset, the AsString should work the some for but components, so if it worked with IBDac it should work the same way for FIBPlus.
You could also use IBExpert to do an insert to the new table in the new database with a select from the old table in the old database and IBExpert will take care of the string conversion. This was not an option for me, because IBExpert would not do this for Interbase. You can find more information on this at http://ibexpert.net/ibe/index.php?n=Doc.ConvertYourFirebirdApplicationsToUnicode