Tacho Generator To Incremental Encoder

Hi all,
I've purchased a cnc mill to rebuild , it has 3 DC Brush Type servo
motors one is Fanuc Yellow Cap.

All Three have Tacho Generator installed on the shafts as feed back
to the drives . Can I replace them with 2 channel incremental Encoders
(Differential / TTL) to be driven by Gecko/Rutex drives ? Would it be
succesful with such type of motors?

Thanks,