You do not have to activate the Lucid Charging Plan upgrade. All you have to do is to go to your lucidmotors.com account, click on the Account Info tab, scroll down, and make sure that you have a valid credit card on file in your Lucid Wallet. That's it. This is exactly what the email sent by Lucid said to do. When you do your free charging, you will see a $50 pre-authorization charge appear on your credit card, but this is a common practice, and will go away after a few days. Of course, if you do manage to leave the car long enough to incur an idle fee, you will be charged for it.
Some forum members think that you also have to activate the upgrade in order to keep your free charging, but you do not. I suppose you can if you want to, but you do not have to. (I don't want to on the off-chance that a handshake problem when I initiate my free charging might possibly default to charging me via the upgrade before I have a chance to resolve the handshake problem via the app.) Hope this helps.