As far as what, if any, affect the upgrade will have on your Domino applications, I guess that would depend on the application. Here is a technote that contains general information on upgrading to Domino 8 as well as some information specific to what you need to do in preparation for the upgrade and after the upgrade is complete. It has some database specific information that I thought might answer some of your questions. It references an upgrade from Domino 7 to 8, but you may still find it helpful.