I have tried this method, as im currently doing this now. You can
however, still use the existing .gdb file in Firebird 1.5.x no problem.

I have build Firebird 1.5 into our InstallShield installation no
problem.

Only thing you need to remember, is

isc4.gdb, isc4.gbk are renamed to security.fdb, and security.fbk

Works 100% fine for me, with no problems.

Also, you can benefit from Aliases in FB 1.5


Problem i have at the moment is with an installation onto a Windows
2003 Server.