BB - I am a newbie and not an expert, but I think that 2 things happen here. One, I read that older bees can revert to raising a queen (providing royal jelly) when needed, and also that in a trap out, when the nectar stops coming in (no foragers returning) the queen will stop laying due to lack of resources and teh nurse bees will eventually leave the hive as well since there is no brood, so they will then start raising anything they find in the box they are all moving into.
At least I think that's how it works. Hopefully one of the experts will chime in and correct me or elaborate. :)