Oh, right, didnt think about it trying to roll when replacing the cups. Maybe try a make-shift wheel chock with some towels or wood wedge, really anything you could jam under the other wheels.

Are you sure the wheels will even damage the floor? Is this a stage, like normal black stage deck? If so, you’ll be fine. If not, and the hardwood is like a really nice finish or dents/scratches easily then there’s likely to be a risk. How was the piano brought in?

I didnt think to ask if the castors were plastic or metal? If they are plastic, you’ll be good. I was thinking about the wide metal castors ive seen a lot. If they are metal, unfortunately i dont have any perfect answers- you’ll have to get creative, i guess. Cardboard, scrap carpet pieces, basically anything to buffer under the castor. But nothing you’d want to reuse ‘cause the castors might damage your makeshift padding.