The Switch to Classic button is only present if you have Microsoft 365 Outlook Classic installed, otherwise, if you just have the New Outlook that ships with W11, you will not see the button. also, It is only Oneway from new to classic - you must relaunch new if you want to use it while also using classic for testing etc.
To update my earlier post on my own testing of Outlook New and Classic vs Outlook 2013, I switched all of my accounts in New from Pop to IMAP and left my primary account in Classic with a Pop due to my existing PST file and switched the other accounts to IMAP since I did not need to store them locally and everything is working fine.
One thing I did notice is how the Classic, New and Web are able to deal with my non-microsoft hosted domain and email - Classic is storing everything locally in a PST, so I have email, Calendars, Notes - the full Outlook pkg, while the New can access the email, it cannot access the calendar since it is stored locally in a PST? (even though I am using the same PC to run all three at the same time) And Web cannot access the email from the 3rd-party hosted mailbox but does somehow manage to access the Calendar from my Classic Outlook, possibly because I am logging in to my PC, Edge and Classic with the same microsoft account. Interestingly, Outlook 2013 is still there, and I can launch it and access the data files, but I can't send or receive, just view. My next step is to remove the 365 trial and return to Outlook 2013 for the time being, as I don't have the $99 to spare for the 365 Family package with the Holidays coming up...