Apparently the other way around. 3.21 had CTD when I tried to load the new turn, so .trn is not compatible, with all the new stuff in that. 2h files, however, should be OK unless some old commands are not valid - which I'd doubt.
Maybe a slight warning/remark. I am playing a sp game (Acre EA) with the CBM 1.41 mod and after patching to 3.23, I played an additional two turns. In both turns I recieved a host message that I was cheating (too much income). Obviously the AI was very upset about it and maybe in multiplayer mode people might be even worse