Hi Jonathan,
The issue with mass updating them back into Leads, if a Sync has completed since - they have already been sync'd over to your QuickBooks file.
Follow THIS help center article on how to delete multiple records from a Grid, to make sure we remove them from both Method and QuickBooks as customers. Using the CreatedDate field you can filter the Grid to show you only those imported since that date.
Be extra careful here and tippy-toe your way around the steps, these are permanent and there is no undos. Do ask as many questions needed.
Once that is finished you can re-import them back as Leads.
- Mortaza