I went from a Harley Custom Sportster with forward controls and drag bars to the FJR. Before the switch, I didn't think I'd enjoy the feet-back position, because I'm quite tall ("Oooooh, Jb, you are Soooo tall...."--that's what all the babes tell me, constantly). I figured my knees would be up in my chest, and in fact the "larger" Harleys I'd sat on with the position-back pegs were uncomfortable. Also, I have a bad lower back.
When I test-rode the FJR, I fell into the position comfortably immediately and never looked back. It's much better than the recliner-couch position. You're much more engaged with the machine and performance.
Now, when I ride the Sporty for longer than a few miles, my back starts to ache. And the Harley suspension (or lack of) adds to the brutality. On top of all that, I do a 5-minute back exercise routine, including sit-ups, every morning before getting on the Blue Rocketship.
In short, no problemo with the transition.
Jb