When you try to upgrade the PRO version of Price List, you can see you'll installed the FREE version. This can have 2 reasons:
1) You entered a wrong Download ID (or leaved it empty)
2) Your subscription is expired
You'll see this error because the PRO version can only be downloaded by subscribers.
How do you solve this?
Go to components > Price List
In the right top you find a "options" button.
Click on it and go to the tab called "Live Update"
Here you can enter your download ID.
Your Download ID can be found on http://www.joomill-extensions.com and if you login go to your account-page. It looks like:
After you entered your download ID and update the extension the same way as the free version.
When you still see the 403 error, make sure you have entered the right Download ID (Every subscription has his own Download ID #) and you have a active subscription. When your subscription is expired you cannot update the extension.