Go to Customers / Account Group

Create your new group

Select the + button to add accounts

Filter by specific bill profile(s)

Select all customers that need to be switched

Add to group

Once these customers are added to the new account group, select the 3 dots & select Change Billing Profile

Select from the dropdown the bill profile to switch all of the accounts to and save.

All accounts in that account group have been switched to the new bill profile.