Hey, sorry, I know this is about a year late, but in the name of documentation.
I loaded a ultimaker 2 replacement feeder motor into my UM2+ and for the most part everything is okay, but there were/are 3 hiccups:
1) The um2's motor shaft is about 10mm too long for the um2+, so I had to add some spacers and get new m3 screws.
2) I had to calibrate the steps per mm of the extruder. This shouldn't be as problematic as it was, but I kept getting really inconsistent data. Like my set steps/mm to extruded distance (when requesting 100mm) was exponential, which was wack, so I basically guess and checked until it was "good enough". That said, there are a lot of factors a play, so it could have been something in my setup (like my spacers or the age of my extruder assembly, ~10000 hours) which wacked out the results.
3) When feeding material in with the "Change Material" function, the motor basically screams at me and stops spinning. I think that's just an under (or perhaps over?) powered motor thing, but I haven't really got a chance to research it enough to mess with the settings. Not sure if changing the motor current would do anything. If anyone knows whats happening, please let me know.
The print quality seems pretty okay though (esp considering the really questionable calibrating job). Its not something I'd recommend yet, but if I figure out the changing material issue and see consistently good print quality, I'll report about it.
P.S. This may be super obvious, but if you're trying to send gcode commands to your printer (i.e. for calibrating, NOT for like printing), I'd really recommend using Pronterface. Theres no console in Cura, so you can't see the printer's responses to commands like M503, which returns the steps/mm calibration of the printer's motors