Mad_Cyril Should be pretty simple. If you can give me the mix file, a copy of the track you want replaced, and time stamps for in/out, I can splice it for you.
Let me know if I have the idea right:
Current mix is Track A > transition A to B > track B > transition B to C > track C
Do you want to remove track B completely, leaving A > A to C > C or is it re-recording a transition and replacing that transition?