Desktops reconfigured on dual monitors after upgrade

First, check System Settings >> Displays and make sure the two monitors are arranged correctly. 

Then go to System Settings >> Desktop & Dock >> Mission Control and make sure Displays have separate Spaces is enabled. 

Apple confirms that this setting provides separate Spaces for each display. Apple: Change Desktop & Dock settings on Mac

Open Mission Control and recreate the arrangement you want: five Spaces on the left display and five on the right. 

You can add or delete Spaces from Mission Control. Also consider turning off Automatically rearrange Spaces based on most recent use so macOS does not automatically change their order. Apple: Work in multiple spaces on Mac

The remaining issue is the keyboard shortcuts. Apple documents keyboard shortcuts for moving between Spaces, but its documentation does not provide a setting for manually assigning specific Space numbers to particular displays. 

Therefore, if macOS 27 is giving the Spaces on your right display numbering that conflicts with the shortcuts you previously used, there is no Apple-documented Mission Control setting that lets you manually assign those numbers to the displays.

So the supported approach is to restore the five-and-five Space arrangement and prevent automatic rearrangement. 

Whether that also restores the exact Space numbering and Control–1 through Control–10 behaviour you had before the upgrade is not something Apple's documentation provides a setting or procedure for.