I don’t think that you can change the username you have – at least the one that will display on the forums here. I’ve looked into this myself in the past since I chose a boring username and had hoped to change it. There was another thread that essentially indicated that it wasn’t possible.
On the page where you subscribe to TPs, just underneath the “subscribe” button there is a link that reads “Own a current Unofficial Guide?” When you click on this, it opens up another portion on the page that allows you to enter some information that would help to confirm that you have the book. Then the discount should be applied to the price.
If you have already subscribed, I would suggest as @brklinck did to e-mail firstname.lastname@example.org to see if they perhaps can refund the difference or work out another solution.