Hello Mary,
If you did that, so long as the swarm had a queen, they would come straight out again leaving the young bees behind to raise more cells from the young larvae left in the brood.
If their queen was lost during the manoeuvre they would stay long enough to raise more cells then they’d be off again with the first virgin to emerge.
The best thing to do is try and prevent swarming from happening in the first place by giving plenty of supers before they need them.
So, if they still want to swarm, and mostly they will want to if they are strong and healthy, then use swarm control such as an artificial swarm to try and fool them into thinking they have swarmed.
Once they have made up their mind there isn’t much that will change it except a strong flow.
Murphy’s much better now thank you. I didn’t think of ice cream – he’d love that! Instead I was soaking his biscuits in bovril. Sloppy fat off a bacon joint went down well too.