I can't get a definitive answer on androidforums, so I figured I might give it a try here.
ATT are bastards and haven't released froyo for their phones yet. My captivate is rooted and I removed(well, renamed to something android ignores) all the bloatware, overclocked the CPU, etc...would I be able to sideload froyo onto it in a rooted state with all the sexy SetCPU profiles I have already, or do I have to remove SetCPU, OneClickLagFix, etc, unroot the phone and do it that way? I'd really prefer not to do the latter because I wouldn't want to have to buy SetCPU again.
You should be able to upgrade it to Froyo with settings intact. I can't say for certain, as my D2 came with 2.2 default. I do know, however, that many pre-Froyo devices were bricked if the user tried to either SBF a previous version or load a previous version's Nandroid backup after upgrading. So it's pretty much a one way street.