The 7.3.4 to 9.2 migration will definitely involve more testing since you're skipping quite a few releases and a lot has changed between your starting release and the target release. If the application (or at least the data access method, i.e. ODBC drivers) using the 7.3.4 database supports the 9.2 database, I would expect that the application will continue to function properly. If the application uses ODBC, for example, you'll need to make sure that the ODBC driver version on each client supports the 9.2 database. It appears from the release notes for the 18.104.22.168.0 ODBC driver that it supports 22.214.171.124.2 through 126.96.36.199.0 systems.
I don't think there's any silver bullet that will answer your questions. In any upgrade of this type, I'd always upgrade a copy of the production database and test the application as thoroughly as you can. There's no substitute for actually doing a test.
This was first published in March 2004