Li Shenron
Legend
So it is as much of a change as tasha's guide?
Seems not worth the 5.5 tag then.
I am not sure, as I said I haven't followed up closely the recent test packs and reveals. I kind of think the changes from Tasha are bigger than the changes between Tasha and 5e, and some people even called Tasha already a 5.x.
I did use 3.0 and 3.5 for quite a while together. I actually did not buy 3.5 books for quite a while, and just used the SRD 3.5. We still used the forgotten realms campaign setting and splat books. But there were quite some points we had to modify, because skill were changed, spells were changed and how small characters worked too.
Well you are saying it yourself that you had quite a bit to change! If you have to change stuff, I wouldn't call it compatibility, otherwise I could say all editions are compatible to some degree, since for example we run old editions adventures all the time.
What I personally remember though, is quite a lot of hostility against using 3.0 material in 3.5 in these very forums, for example if you wanted to play PbP games they would normally just require to stick with official 3.5 material. Maybe this time will be different but I doubt.
I guess, using 5.0 material for a while would work way easier, until all former options are replaced by newer ones.
Probably easier than 3.0/3.5.