You are talking about a LOT of trust in the step-by-step upgrade from R1/T1 to 3.0, then to 6.1, and then to 7.0/7.1 (I may have missed a step in there...)
In my experience, especially if you have a lot of programming (like DID's, etc), you will be better off copying down the programming and going directly to the newest version of software. You will certainly need a new voicemail, as the VM that is R1/T1 compatable is not 7.0 compatable. As PERRYPJ said, GOOD LUCK!