QuickBooks is currently rolling out Bill Pay powered by Melio to QuickBooks Online (QBO) users. You can easily identify your clients that have Bill Pay powered by Melio enabled, because on the "bills" screen in QBO you'll see "schedule payment" buttons. Like this one:
If you have a client's QBO account that has yet to have the new Bill Pay powered by Melio enabled. You do not need to do anything, they should be getting it very soon.