Update, one month later:

I'm really liking this new position. My butt hurts less, my hams are worked more, and the dramatically different muscles worked between sitting and standing means that I can get out of the saddle at any time and feel fresh as if my quads have hardly been working. This came in handy at my first cyclocross race, when I had to push up short steep hills, I felt good doing it out of the saddle. And on our group rides, going in and out of the saddle made climbs much easier. (In the past, with my old saddle position, standing up was hard as soon as I stood, cuz my quads were already tired from seated climbing.)

Regarding muscle retraining, my hamstrings are gaining the ability to compensate and adapt for the new work load, so I'm happy with that also. A real coach/fitter might say I'm working them too much when seated but it seems to be working for me.