A few years back I have bought a couple of dual opteron MB- Tyan's 2885.
Since Opterons were very expensive then, I took for each board cheapest 240s with the intention of upgrading them when 2xx series matures.
This time is slowly coming, but now Tyan says that 2885s from any but las PCB revision support only basic 240 model, even though at the time of the purchase Tyan declard support for every opteron and even listed everything up to 248, which was latest model at the time !!!
Tyan says that it isn't their fault and that they had to make some changes because AMD ahs changed specs of new chips. That is all I have got from Tyan and the refused to clarify specifics, but many people report succes with new chips and old PCB revisions, so I suspect the only change is slightly higher power envelope (95 vs 89W).
I am proficient with electronics and would make changes myself rather than throw 2885s with 240s away - MOSFET, ferrite beads etc replacement is no problem...
You can insert all "old" Opteron CPUs with B3, C0 and CG stepping. Just Ex-steppings require the newest board revision.
And even this is not 100% correct because I know some people that have old board revisions and do use E6-stepping Opterons (like the 275HE or the 265) that will not reach the 95W TDP of the new CPUs.
So it may be hard to buy "old" Opterons but you should be able to upgrade to two 248s e.g. without ANY problems and it should be save to upgrade to 275HEs as well - with the risk of little complications BUT: The HEs have a lower TDP than 89W so there will be no problem on the electronics in terms of the "required power".
I have 246ers on my S2885. I bought the CPUs at the same time and thought several times about upgrading to HE dual-cores myself. I would not recommend to buy plain 280s or 285s because these CPUs may max out the 95W TDP and put too much load on the VRMs on the board.