> I've changed a field collate, but it only "worked" after a backup and restore.
> So is it really required to backup and restore the database when a
> collate is changed?

It is a bug, present even in FB2.1.
I already put it into tracker 3 months ago.
(http://tracker.firebirdsql.org/browse/CORE-1432)

As a workaround, just do dummy update of the table.

Ivan P.