changed gamer tag now can't make purchases from WOT Store

If you can't seem to figure out the issue you could always try going to a retail store and purchasing an Xbox card which would also work if he's changing his gamer tag.

Gamer tags can't. Be deleted but can be name changed. You can also make a new account for free but you will loose all gamer points. To change a gamer tag it costs 800 Microsoft points and you keep all your achivements and gamers score

I have made my own gamer-tag which was ( BIgBoyLuVMilPhZ ) and they forced changed it which was pretty dumb, and now I have to make another new gamer-tag with my money again. I think they should keep that (FREE NAME CHANGE) for the people who get there gamer-tag
banned for the first time. Don't you think? :/

I would check to make sure you upgraded to gold with the correct email address. Normally, your gamer tag won't change when purchasing gold. In fact, you either have to change the gamer tag via an 800MSP purchase, or a forced change from the XBLPET.

I would log into your normal gamer tag on your console and then check to see what the email address should be. In the upper right hand corner of the screen, your gamer tag will roll over to your email address. If it doesn't, then you can change a setting
in your account to get the Microsoft Email Address to show. It should be in your profile settings under 'Show Microsoft Account'.

If that still doesn't work, you can always call support at 1-800-4-MYXBOX, and they can aid you in confirming what the email address for your gamer tag should be.

I hope this helps!

People should get a new change gamer tag as people change and gamer tags may be irrelevant now.