The reason you don't see the "Upgrade" button anymore is simple: you've already reached the standard level. Once you move from "Free Trial" to "Pay-As-You-Go," you are on a full account. In the eyes of the portal, there is nothing left to "upgrade" to because you now have access to every service without the trial restrictions.
Quick checklist to be sure:
Check the Status: Go to Subscriptions. If it says "Active" and the offer is "Pay-As-You-Go," you are all set.
Don't worry about the button: It usually disappears because its job is done.
Wait a bit: If you literally just did it, the portal might need a few hours to refresh the dashboard view. https://azure.microsoft.com/en-us/pricing/offers/ms-azr-0003p/ .If this cleared things up for you, please upvote the answer!