When he moves in you have to ring and tell them. Your single claim will end and a joint claim started.
I just did it the other way around. Went from a joint claim to a single.
Told them and they ended one and started the new one almost immediately.
No gaps in payment. Same payment day the next month.
I guess it all depends on what changes you have to make and when your next payment would have been due?
You can change everything on your online account and send them a message on your journal. Ask every question on there and also write down on the journal every change you make and everything they have told you to change! It keeps a record and shows you’ve done it all right and if they make a mistake or underpay you there’s proof and can be sorted faster.