Q: how to change ur account from using visa card to iTunes card

You have your US billing address on your account ? If you still have your UK address then you will need to change it to your US address, you have to be here in the UK to use the UK store.

Changing country : Change the country of your iTunes Store, App Store, iBooks Store, and Mac App Store account.

For your card, on your computer's iTunes you should be able to edit/remove your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should be a payment link.  If you are doing it on your phone then tap on your id in Settings > iTunes & App Store and tap on 'View Apple ID' on the popup and log into your account  - that should also give you a payments link on your account's page.

Changing payment info : http://support.apple.com/kb/HT1918

If you don't get the 'none' option on the payment details screen : http://support.apple.com/kb/TS5366

And have you added your US payment details ? From the last link in my above reply :

If you changed your country or region

When you change the country or region of an existing Apple ID, you must provide a payment method and update your billing options. If you want to remove your payment method after you change the country or region, you can change your payment information to None.

What happens when you try to change it ? The steps on this page should work : Change the country of your iTunes Store, App Store, iBooks Store, and Mac App Store account

But as part of changing it you may need to enter your US payment method, if you are then you should be able to remove it after entering it.