> This is probably a stupid question but here goes anyway. I have a Delphi application that is currently running on FB1.5. I need to move to FB2.5. Is it necessary to update the FB client on the client PC's from 1.5 to 2.5 or is the 1.5 client compatible with the 2.5 database?
> I have found that PCs that have the 2.5 client installed connect happily to a 1.5 database.

You can use the old fbclient.dll against the new server. Also you can
replace the old fbclient.dll with the new one, they should be binary
compatible.

Problems may arise since the Server is not fully compatible - SQL
working on 1.5 might fail on 2.5.

Elmar