How can I move several transactions at once from one Vendor/Customer to another?

Thanks for posting your question here in the Community forum, @jo_eaton.

You can change the Customer/Vendor Name in QuickBooks Desktop in which all the transactions under that customer/vendor will update.

To change a Customer's name, here's how:

1. At the top menu, click Customers.
2. Select Customer Center. 
3. In the Customer Center window, go to the Customers &Jobs tab.
4. Locate and double click the name of the customer. 
5. In the pop-up window, change the customer name.
6. Click OK. 

To change a Vendor's name, here's how:

1. At the top menu, click Vendors.
2. Select Vendor Center. 
3. In the Vendor Center window, go to the Vendors tab.
4. Locate and double click the name of the vendor. 
5. In the pop-up window, change the vendor name.
6. Click OK. 

But, if you're trying to move transactions from a customer to vendor, then it's not possible.

To learn more about this process, you can read this article: Change vendor, customer or employee name type.

You can get back to me anytime you have additional questions in QuickBooks by posting your response below. I'll do whatever it takes to ensure your concerns are addressed. Have a great day.