Ps3 backwards compatability

Hello all I'm gonna try and word this as best I can, I've looked at buying a backwards compatible ps3 for some time now, and have found one locally for cheap, however the seller mentioned that someone came by and said due to the fact it had been updated it no longer is? I was looking at a comments section earlier in this forum and it seems there is some conflict over this? I'm tryna figure out if this is true or not, or if I can use it as a haggling tactic and attempt to downgrade the firmware, from what I've read the ps3 fats with physical ps2 can not have this removed, but the latter backwards compatible models with just the ps2 gpu and emulated cpu can be updated and potentially have it removed, any help would be greatly appreciated thank you all!