Lia D'Antonio (LiaD'Antonio) 

First of all I guess this was an old purchase since it hasn't been possible to purchase 2021 since 2022 was released.  Just checking!

Lia D'Antonio said:

What happens if I press Process Upgrade?

Your 2017 activation code will be removed and you will receive a new licence that will only work with 2021.

Lia D'Antonio said:

Will there be only Studio 2021 running on my PC with a licence with no expiry date?

Processing the upgrade has nothing to do with what's on your PC.  You can download and install 2021 alongside 2017 and run both on your computer.  But only 2021 will have an active licence (with no expiry date... unless you purchased an annual subscription).

Lia D'Antonio said:

Does it mean the Deactivation/Activation tab within Studio 2017 installed on my PC?

Yes.  If you leave 2017 alone then it will continue to work.  However, you will not be able to transfer 2017 to a new computer; and if you do require a licence reset in the future you will not be able to get one as the licence won't exist anymore.

Thank you for your prompt and detailed reply. I bought the upgrade to Studio 2021 in November 2021.