We have been using IBO and Firebird for several years in a product
that we release to our customers each Fall. Some customers have
multiple releases (each from a different year) of our product running
on the same computer. Some of the older releases use IBO 4.2lb and
the newer versions use IBO 4.3Aa. All releases use Firebird version
1.0.3 and everything works great.

For this year we considering upgrading to IBO 4.5B and Firebird 1.5.3.
The basic problem is we don't want to break the older releases by
upgrading the Firebird server. I understand that the releases with
IBO 4.2lb will require the OldParameterOrdering setting to be set to 1
to work properly. Will the newer releases, using IBO 4.3Aa and 4.5B,
have any difficulties with OldParameterOrdering set to 1?

Secondly, I understand that if we change to Firebird 1.5.3 that all
clients should use the 1.5 fbclient.dll (which we will rename to
gds32.dll). But some clients will undoubtingly miss this step and try
to connect using the old 1.0 gds32.dll. Will they get some kind of
connection error? Will any data corruption happen if they can connect?

My understanding is Firebird 1.5.3 can work with database files
originally created with Firebird 1.0. If this is so, we will not have
to convert the existing databases by backing up under 1.0 and
restoring under 1.5.3. Is this correct?

Any other advice for our situation?

Thanks in advance for any assistance.