Lola That's ultimately up to you how exactly you do it. Some people make use of the Overstapservice when they switch bank accounts, some don't. Some people cancel their old account right away, some keep it active for a certain amount of time, or just use multiple bank accounts permanently. Whatever way you choose can have certain advantages and disadvantages.
For Overstapservice: contact ABN.
To cancel your paid plan at bunq: Profile tab > Settings (top right) > My plan.