I did some looking around before doing this but, it can be done. The best part is it was complete plug and play, no moving pins or getting a special harness adapter.

Pulled my buddies H/K from a 2012 FLHX and put it in another buddies 1999 Electra glide. Everything works except for the AVC. All hand control's work just fine.
***Remember to swap out speakers though as the 1999 had 8 Ohm stock speakers. Put Boom speakers in and it sounds great, a night and day difference over the old cassette player and paper tweeters..

 

That's pretty awesome it swaps that easily, though from what I've seen and heard, the updated hk stereo is inevitable mediocre. I'm currently contemplating installing a Sony head unit next year, because even the biketronics amp/speaker package I have installed just doesn't compensate the poor sound from the hk unit.