Taking queen size box spring to a another floor?

Sell the box spring and buy a bed frame that doesn't need one. There is no magic way to reduce the size of the current bed frame unless you cut it up. I live in an old house, we bought a low profile split box spring. We've replaced the mattress but keep the same box spring.

Had the same issue in a '30s era house. Couldn't get the box spring through the top floor stair opening.

As above would be the best (boxless bed frame). Cut a box spring in two and it will probably lose all structural integrity and strength, not that there is that much.

Queen sized split box spring is what you need

Yep - no answer to the problem. It is what it is.

Our King mattress is on the third floor. I took it up there when I was rebuilding the stairs, so it was lifted straight up into the room. No box spring though - it's on a simple IKEA bedframe with slats and a headboard I made myself.

When it's time for it to go, I suspect it will need to be bent beyond limits to get down the steep 100 year old stairs. It would likely get replaced with two Twin XL's.

But it's a great mattress and I have no doubt I'll have it another 10+ years.

Many older houses have strange windows, tiny doors, or other openings on the second or third floor designed specifically to move in mattresses.