Switch from 'new' Outlook (2023) to 'classic' outlook

Once flipped on, the toggle is irrelevant since the "old" version still exists. Just run it. You should still be able to find its shortcut in Start (and wherever else you might have had it before), but if not you will see outlook.exe in one of these places (may vary slightly depending on edition), so if necessary make a new shortcut:

C:\Program Files\Microsoft Office\root\Office16

or here:

C:\Program Files (x86)\Microsoft Office\root\Office16

The nice thing about the New Outlook is that it's not an either/or proposition (like New Teams is). You have both old and new. You can even run them concurrently.

But if running that actually runs New Outlook for you (some have reported that's the case), then either:

1. Uninstall Outlook

or

2. run Regedit.exe and go here (or the same with 15.0, if you don't have 16.0):

HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Preferences

And in that you will see UseNewOutlook

Set it to 0, which means that running old Outlook actually runs it.