Yep, to clarify in more laymans terms, there are 2 options, a cash upgrade option, or an upfare.

The cash upgrade option will appear in app (or sometimes on the website) and will be dynamic based on some random calculation. (In the app, I have to refresh the booking to get it to show sometimes.) Doing so will charge a card of your choosing and is basically just a seat selection fee. You will get the extra bag benefits, boarding, etc as well, but will not earn extra miles/LPs (this likely doesn't matter for you). If the trip is cancelled, you will have to request separately for credit, and it will be returned separately.

If you call in, you will get an upfare. The price will still be dynamic, but will be based on the difference between price that was paid for your current seat and the current price of the upgraded seat. You will get all the benefits, etc, as well as increased mileage earnings based on the new fare paid (and not the original fare paid). If you cancel, it will all come back as one credit.