It can be done, but I've never really understood why people worry about it so much. We do anywhere from 4-6 SHO motor swaps a month, and if the A/C was working when they came in, it's working when they leave (and we purge and disconnect the lines). We've never had a come back for an A/C problem afterwards.
IMHO, it's just a lot of worrying for no reason. Just purge the system, disconnect and cap/plug the lines. For us, every time-saving step is worthwhile. If you have the time and it's really important to you, then go for trying to keep it intact, but again, IMHO, it's a waste of time.