I know this was by design, but let me explain why I think this is a flaw.

I have max population (200) and I want to move one of my barracks. There are plenty of reasons to want to move a room, in my case I want to put similar production next to each other and this room was in the way. In order to accomplish this, I have to destroy the existing one and build a new one. I would even be happy with the option to build a new one, then destroy the old one, but both of those options are unavailable.

The only way is to kill off dwellers until my population is low enough to destroy a barracks (that means killing 40 dwellers)!

I request that we would be able to build and upgrade additional living quarters even after we've hit max population.

P.S. I did decide to kill off my dwellers to get past this, and I noticed a related bug. Based on how I upgraded to my last barracks my "theoretical" max capacity was 202. I figured that that meant I only had to get my pop down to 162 in order to delete a barracks, but it still did not let me delete it. After I got down to 160 and deleted the barracks it said my population was 160/162, so I know I didn't calculate it wrong.