10 Magic cruises...3 upgrades. Each time, it was different. ONce, it was a call from the TA before documents--like 2 months out saying that we were "offered" an upgrade, did we want it? Another time, it was an upgrade reflected in the documents that I knew nothing about (but identified on the cabin number on the docs). The third time, it was at the port--the luggage porter had a diffrent cabin number than I had, and he hinted that it was a good thing. At check in, the CM said something like "we've moved you to XXXX. It's on the same deck but is a little nicer stateroom (like from an 11 to an 8 or 9 in that case).
So...upgrades come sporadically and in different ways. The rule remains to book a cabin where you will be happy. If you happen to get an upgrade, you rejoice.
This is purely my observation and may not be true...but it seems that in my case, an upgrade is a little more likely if you booked a gty room....and maybe if you booked farther in advance. We've never been upgraded on a "close to sailing date" booking. Also, off season seems to be a better chance than prime season.