Thank you for posting, @Roving.
I'd be glad to help share some information about making a backup file from your old business.
QuickBooks will save your backup data as a disk image file, you can restore the company file and attached documents by clicking the .dmg file. You can check out this article for more information: Restore a backup (.dmg) in QuickBooks Mac.
It'd be best if you make two backup copies from the old business. This way, you'll have an extra copy just in case.
As always, you can contact our QuickBooks Mac support if you need further assistance in going through the steps.
Here's how you can reach them:
That should do it. Let me know if you have additional questions about restoring the backup. I'm here to help however I can.