You go up the stairs and the last dungeon floor is gone forever.
This is fine for a 7drl but it leaves out a lot of game play possibilities that I really wanted to have in the game, like fleeing upstairs from strong or out of depth monsters then coming back when you're stronger and beating the crap out of them. Or having things on lower levels you can't get to but on level 5 you find something that lets you get to it. That sort of thing.
The easiest, laziest way possible. Just make objects not on your floor invisible and leave them out of game mechanics. Boom. Now you can build 10 floors but it will only show the stuff on your floor. No need to store anything, or re-build dungeon floors or anything complicated like that.
It has the added advantage of having all the entities on all floors there all the time. Some maybe minor interactions could happen on floors you're not on or more easily be able to chase you from floor to floor.
But whenever you "tack on" a big thing like that, there are issues. For example I would see monsters on floor 2 that should be on floor 1. Like ghosts, they just sit there and are intangible because I 'leave them out" of game mechanics, but there they are. This is because I had a line of sight function that hid and revealed monsters if they were behind walls or not. That function wasn't floor savvy so it would "reveal" monsters I had line of sight to on wrong floors.
There's bound to be stuff like that when you add features late in development without starting over, but I think I got it licked.
I can now go upstairs to higher floors without destroying the lower ones, and it seems to work seamlessly and with no side-effects of having all the lower floor objects still exist.
Another fun side effect of this is now I can have basement levels! I chose to progress UP instead of down so I could keep those cool stained glass windows. But now I can have basement and cave levels so more variety is possible.