One idea we had discussed it to provide with the V6 distribution a version
of gbak.exe with the Classic client library _statically_ linked in. The
Classic access method includes all the code needed for local I/O to a V5.x
database.

So once you install IB6, and can't access the database, the answer is:
"run interbase/bin/gbak_v5.exe -b old_database.gdb database.gbk, then
interbase/bin/gbak.exe -c database.gbk new_database.gdb"

No reinstallation of V5.x is necessary with this plan.

Bill Karwin