Even if the used shaft is .005/.010" undersize at the joint to the cue that you want it to fit, it can still be brought to size through changing the ring-work and building the diameter with finish. Any pin config. can be changed as well.
One of the conversions that I would try to avoid would be trying to match a .840/,850" shaft to an older Joe Pechauer joint which is .880" nominal. That's just too damn much finish. I had to do one once and when I was done I made the comment that if the client ever dropped the shaft, the finish at the joint would probably shatter like glass. LoL.
Bottom line, if you have a shaft that you particularly like, it can be modified to fit just about anything.