I've been upgraded a couple of times at my local Garden Inn to an accessible room. Since it's usually a Jr. Suite, I don't complain.
The only difference I note is that the shower is the walk-in variety (and I love it) and there's a bit more room in the suite.
I would never book one (I've never even seen one offered at a lower rate -- maybe because I don't look for them), but I assume that Hilton knows what it's doing when they assign me the room.
If they asked me to to move I would do it in a heartbeat.