Time Machine Query

I have had my MBP for a few weeks and cant believe how much better it is than any PC I have ever used, I wont be returning to the darkside (windows).

I have invested in an Airport extreme to take advantage of the ability to attach an external hard drive for my time machine backups. This is a brilliant set up and one that takes all the hassle out of keeping my back ups in order. Anyway I am looking to upgrade the hard drive that I have attached to a 1 or 2 GB and was wondering how easy it is to trf the existing time machine files I have already have saved to this new drive and are there any 'how to's ' online anywhere ?

Thanks in advance for any tips or advice you can give.