The rules say that you can add a

to any

in play, and if the number of slots has already been reached then you are supposed to swap out an existing one for a new one.
Do you have to fill up all the slots on all

in play before you can start swapping out

? I think that's the intent, otherwise you could just keep swapping out a

on a

that only has one slot over and over without ever adding any more to other

, apart from the ones that were added during set-up.
But since the rules don't explicitly say that (unless I missed something), I just wanted a clarification.
Thanks for your time.