Hello adezyanian,
Since your client already allows to transfer their billing, all you can do is add them to your subscriptions. I'll guide you to complete the transfer with these steps.
- In QuickBooks Online Accountant, go to Settings ⚙ and select Subscriptions and billing.
- Select the Your subscriptions tab.
- Select Add existing clients.
- Select the checkbox for your clients you want to add.
- Select Add clients.
You can refer to this article: Transfer clients to your wholesale discount plan. There is also an alternative method in Step 3.
I'm just right here if you need more help with the billing transfer.