AFAIK, Interbase has this feature. You can open older ODS database
using newer version of IB. It allows to upgrade the database server
first, then use (new version) of GBAK to backup/restore the (old ODS)
database to upgrade the ODS. Is is even possible to connect to old
ODS database and work with it (without using the new features). Does
anybody know whether this behaviour will be possible in future
versions of FB ?

I know the best way to upgrade to a new version of IB/FB is to make
the backup first, then reinstall the server and finally restore the
backup. From the end user perspective, however, it is easier just to
upgrade the server and then let the application program to upgrade
all its databases.

Thanks,
Michal