I haven't tried changing on the fly. I think it's fairly unlikely that it would just work, because you need to create all the database tables (at a miniumum), so you would need to re-run at least part of the config tool. You'd also need to install the correct jdbc drivers, modify the DataSource file, etc, etc.
My suggestion would be to bite the bullet and re-install - I think you might find it easier in the long run.
Specializing in Custom QPAC development and LiveCycle Solutions
I have try, I use the PDF manual come with the software but is not working. When you say "insttall the correct jdbc..." where you take this information because in the manual i dont have it. i have only to create a file call adobe-ds.xml with some information and they dont speak about jdbc etc...