When you say "modern encoders", nothing's changed in the meantime. You just need to know if your encoders are single-ended or differential output and which of those your drives require, using a converter if necessary. My SEM motors from the early 80s had Heidenhain encoders with differential outputs that wired directly into the cncdrive DG4S-16035 drives - quick and easy, no messing.

The main difference between DC drives today and 20 odd years ago is that now they use microcontrollers rather than analogue circuits. So you can throw away the tachogenerator and simply use an encoder. Conversely, an actual resolver would be a PITA that you'd want to replace with an encoder. Resolvers give some measure of absolute position, whereas encoders only tell you how many clicks the motor has moved. Again, my SEM motors had tachos but I simply left them in place, as removing them would have involved additional ballache for no obvious benefit.