Apple Store Help

On Wednesday I pre-ordered the 32GB iPod Touch 4th-Generation, and today I have decided to buy the 64GB instead. Is there a way to upgrade this on the website without going to cancel order, getting a refund, then them charging me the extra $100?

I'm worried they might charge me $430 for the 64GB before they refund the $330 for the 32GB and I'll overdraft on my bank account so I'm wondering how it all works and if an upgrade is possible. Despite ordering within 30 minutes of it going on the website, and choosing next day shipping, I'm having to wait an extra week to get my device x-x [/rant]

Any help is appreciated,
Chris