The old nuts should work fine. However You will want to very carefully compare some dimensions from new toilet to existing projections from the wall. Make sure that the drain barrel isn't going to touch the porcelain. Not only because touching will lead to breaking it also prevents the seal from having the right "squish". The bolts may need to be adjusted if the porcelain is deeper. Also, the toilet shouldn't contact or be supported against the wall structure itself, even though a lot of old installations did. A correct installation will have nuts and washers just about an 1/8th in proud of the finished surface for the toilet to rest on. Having those in perfect plane is imperitive.