Reapportionment's gonna be wild if this happens. A lot of states are going to be annoyed at losing/not gaining as many seats as they hoped for.
I mean, Texas isn't going to mind getting 38 rather than 39, but Montana will definitely mind getting 1 rather than 2.
Reapportionment should happen before PR is added. At the very least I expect Democrats to moderately expand the house size by a bit to not piss of any state/delegation.
That's already what the current iteration of the DC admission legislation has (permanent expansion to 436 seats). The one introduced in the last Congress would have reduced it back to 435, so I assume they figured out it would ruffle some feathers if they didn't add a seat.