How to change database dialect?
While you could simply change a flag in database file it isn't recommended as there's much more to it. Different dialects have different ways of handling numeric and date operations, which affects all object that are compiled into BLR (stored procedures, triggers, views, computed fields, etc.) Fixing all that on-the-fly would be very hard, so the recommended way is to create a new database and copy the data. You can easily extract the existing database structure using isql (see FAQ #73) and then copy the data using some of the tools listed in FAQ #86.
It is not possible to change dialect with backup and subsequent restore.