Level 1 to Level 2 Upgrade via PayPal Subscription Modif

I want to clearly understand from an s2Member functionality perspective how existing member upgrading works now to address the new/recent PayPal Subscription Modification Button update.

In s2Member (as of release 190822), with an existing Level #1 Member who wants to upgrade to Level #2…how does/should it work now?

Is it that they must click the s2Member-generated PayPal Subsc Cancel “Unsubscribe” button code that we copy/paste into a page? Or must the member physically log into their PayPal account and unsubscribe within PayPal? Or can they still just click on a “Subscribe” button for Level #2/#3/etc (but with the subscription button code using modify=0)?

What is the easiest and/or most user-friendly route/flow to allow member upgrading (and downgrading) as it once was with the modify=1 / level=1 variables you could add to the buttons.

Please explain. Thank you.

with paypal buttons - clearly first cancelling the subscription from within paypal account is needed. With pro-forms you can use the cancel button by s2member (i guess so at least - do not use paypal pro/paypal pro-forms - but that is the way it works for stripe).
Then subscribe newly.
To do that in one step (for pro-form payments) I guess you would need to have some custom javascript code doing both in one step.