just to ensure there is an answer for this question at this platform I'm adding one :).
If a customer service makes Universal ID mandatory the customers will have to have an Universal ID. But it's pretty easy and quick to setup an Universal ID. Once you have done so you can link any P-/S-User associated to an email you have proven ownership for at Universal ID.
So most customers will either not notice the switch if they already have an Universal ID. Or be quickly asked to create one in less than 5 minutes.
The overall purpose of Universal ID is to simplify account management and improve customer experience.
What some customers do struggle with is that they do not have access to some of their S-User's emails. Which means they are unable to link the S-User to their Universal ID. And thus cannot continue using the S-User with Universal ID. At which point either the partner admin or support will need to help them updating the S-User's email.
To confirm if you are all set up for Universal ID I recommend to quickly create one at https://account.sap.com/core/create/landing . Add all your emails ("add accounts"). And then check that you see all P-/S-Users listed as link-able accounts.
As long as you do not link an account to your Universal ID you are free to continue using the account as is. If you link an account you will have to use Universal ID at most services to access the given account! Or reach out to support to get the account un-linked again 😄