That is correct, you can’t run applications in client-server mode in Forms 10g.
Now, an Application Server is needed.
You can test/run your forms from Forms Builder without the app server, but to deploy them you will need it.
There are some things that could need to be changed in your forms, for examlpe, AciveX, OCX, OLE or VBX controls are not supported and need to be replaced, mouse movement triggers are not supported either, and some built-ins or packages run in the server side only (TEXT_IO for example).
Have a look at the following guide. It may help. Oracle® Forms Migrating Forms Applications From Forms 6i to 10g (10.1.2.0.2) for Windows and UNIX