You can easily "up-fare" on the website. DO NOT CLICK UPGRADE. Of course you can also call

Let's say you're on a round trip. Go to change flight, for each leg click the "change this flight" button but don't change the origin, destination, or date.
Then, you'll see a list of flights on the same dates as your original trip. If you're booked in Economy, it will show "not available" for the economy seats on that flight, but should show an ability to book to First if there are seats available. The dollar amount displayed is the fare difference In this way, you're not upgrading, you're changing your ticket to a regular first class fare. This should provide normal PQP, GS $, and all other credits as if you'd bought First outright.

In my experience, the fare difference through upfaring is the same or very similar to what they offer as upgrade offers. I never purchase the upgrades and instead up-fare via the method above.

United does give PQP for upgrades, but it's not the same as the above. It's a separate process which you can see because upgrade PQPs hit your account several days after travel.