As already mentioned, it is not common. But, I managed to fly SH-60Bs, SH-60Fs and HH-60Hs in the Navy without ever actually changing communities. Rules and policies have changed (or so I am told), so it would be much more difficult to do the same these days.
In the Army? A different story. As we transitioned to the 60M, I kept current and qualified in the UH-60A, UH-60L, UH-60M, and HH-60M for about 2 years. After that there was some reason or another every year and a half or so for me to regain currency in the 60A/L. But for us, it was just a currency flight of 1-2 hours. My least favorite was getting a refresher flight in the UH-60A, after not having flown one for 2 years, and going straight out to the UH-60A instructor course. THAT was not fun. Imagine being evaluated as an instructor in an aircraft you hadn't flown in 2 years, with a completely different (yet similar) -10 (NATOPS manual).
At this point, after having not flown an A/L in more than 5 years, I doubt any ATP commander would set me loose in an A/L after only a 2.0 hr refresher flight. However, switching from the UH-60M to the HH-60M is seamless. I do that about once a week. No refresher flight required.