Import Breakout Rooms but allow participants to move freely

I am designing a Zoom Reunion and want to import the breakout rooms and select the option to allow meeting participants to join/unjoin on their own. These 2 features  appear to be mutually exclusive. When I import breakout rooms, (before meeting starts), it seems to require pre-assignments and then, as host, does not allow me to change to option of having participants join on their own. If I start a meeting with no rooms, and then create them, I do have the option of letting participants manage on their own. I'm trying to save myself the effort of typing in all of the rooms after I've started the meeting. Am I missing something?