I want to link DD payments, but all are already active DD's prior to QB's startup, so do I need to send a DD mandate to activate in QB's?

Hello, office-pvbusines!

Good thing you already have mandates. You can import them into QuickBooks so that your customers' future invoices will have a Direct Debit option. See the detailed steps in this article: Transfer direct debit mandates from GoCardless to QuickBooks Online.

I'm also including these articles for your additional reference:

• Get started with GoCardless in QuickBooks Online
• Set up direct debit mandates for customers (GoCardless) in QuickBooks Online
• GoCardless FAQs in QBO

You can always go back to this thread if you have any other questions about the Direct Debit setup in QBO. Take care and have a good one!