I have a Bridgeport R2E4 that I retrofitted and ran with Mach 2, now Mach 3. It runs beautifully. The only problem I have is the servo drives I used were to small. Mine is a servo machine and I reused all the originall power supplies, motors, encoders, etc. I went with Rutex drives (no connection to the company, just an extremely satisfied user). This is where my only problem came in. I did the retro about 3 years ago now, and Rutex had not released the R90 drive for 200V 40amp. I ended going with, if I remember right, the R990H2 so I would not have to mess with altering/building power supplies. These drives were rated for up to 200V, but only 10amps. Where as the alternative Rutex drive at the time was 100V 20amp. Being lazy (or more cheap than anything

) I was intent on reusing as much as I could, including the power supply. And I did not want to mess with electronice and what not to drop the voltage to an acceptable level for a 100V drive. So I traded off amperage. This made it hard to tune in the drives to the motors since they were soo underpowered. I don't remember the amp rating for the motors or what the supply originally put out to the original drives, but it was more than 10 amps. I had to max out the current limit pot on the drives I had. This underpower condition meant I had to run things much slower than the machine originally could. I run with 100ipm rapids reliably as opposed to, I believe, the original drives 250ipm rapid capability. Slower than original but not too shabby. So the only real problem I had was being a cheapskate. The Rutex drives functioned beautifully, and the tuning software was very easy to use. I now have 3 of the new 2000 series drives on the way from Rutex, the R2020 rated for 200V 40amps. These should bring things back up to speed. Not sure if your mill is servo or not, I don't know if all the R2E4s were the same. Mine had a BOSS 10 control. But mice had done some "customization" of the original wiring with their teeth, so retrofit it was. Mine is also a Series II, again, not sure about all the R2E4s.
Anyway, to make an already long post a little longer

, I would say go for it. If you don't like Rutex, there are many other great drive manufacturers out there like Gecko. I have used them too, and they are also very nice. But if yours is like mine, Rutex was the only one that came close to the electrical requirements. So dive in. I will be glad to answer any questions you may have, to the best of my knowledge and experience anyway. And if I don't know, some one else here certainly will. Good luck and hope that helps.
p.s. I would recommend going with Mach 3 over 2 at this point.