Question: Can FSMO roles be switched over a period of time? or do I have to do them all at once, same day? I wanted to move a role, wait, see if anything blows up, wait a few days, do another one.

I have a forest root that is on it's last leg, but I don't want to destroy everything. Thanks. Windows 2000 Forest and Domain.

Answer: 
Each FSMO role is more or less independent of the other roles. If you had enough DCs, you could assign a role individually to each of them if you wanted, though for a variety of reasons, you probably don't want to do this.

So yes, your plan sounds reasonable.

In fact, I've actually never heard of or experienced issues with moving roles around. As long as you make sure your domain controllers are healthy and that replication is happening as it should, you shouldn't see any issues.
Yes they can but with some caveats depending on your domain configuration and placement of Global Catalogs.

Refer to: <http://support.microsoft.com/kb/223346> for how to decide what goes where and why.