<font size="2" face="Verdana, Arial, Helvetica">Are you saying functional testing of an application during the OS upgrade, or functional testing of the OS? Either way, I would do the full regression as well as performance and Fail-over tests.
Success is the ability to go from one failure to another with no loss of enthusiasm.
~ Winston Churchill ~
If it were me... I'd be asking this question of the product manager, not a group of strangers. [img]images/icons/wink.gif[/img]
That said. I've gone both ways on this issue in the past. First off, chat with your developers and get a feel for how closely the application under test is tied to the operating system... the more it interacts with the O/S, the higher your desire to completely regression test should be.
If the application lives almost completely apart from the O/S, you could get a little bit more complacent and pass up doing a complete regression test. Just be sure to hit the security areas that are going to be changed.
It's all about how comfortable your team is with the decision... if it's only your neck on the line, go for the full test! If someone else owns the final decision, give them all the info you can and let them make the call.
If it were me... I'd point to the sign on my wall, "There's no QA in team." We're all responsible for the quality of the products we create as a team... so we all need to have a chance to speak and help make the decision.