The problem I'm having is that I would like to have trial

Hi guys,

The problem I'm having is that I would like to have trial users who can then upgrade to a premium version. I'm having a problem with this. The steps I've taken was:
1. Register as a trial user on the site
2. Login as a trial user
3. Goto through the account page to upgrade subscription
4. Land on the sandbox paypal site and pay successfully.
5. Return to my website and view my welcome page.
Once I goto my site then it seems it didn't upgrade my account to 'Premium'. When I check the account page it says im still signed in as a Trial user and there is still 29 days to go until it expires.
How can I allow the user to upgrade their package?