No, not bad form. I think it's quite nice of the hotel to offer you the upgrade. Yes, moving can be a pain, especially after you've been in your room for a few days, but my guess would be that most everyone would trade the pain of moving for a nicer room for part of their stay (either end of it)... myself included!

I was just completing a 3 night stay at a Marriott property, and in the evening of my last night a really loud family moved in to the room next to mine - they were incredibly loud. I went down to the front desk and politely asked to move rooms, and the moved me down the hall (from 910 to 904)... which made moving a breeze. Sometimes we happily move for other reasons...