Upgrading to iPhone 3GS

Browsing the O2 website, I found out that existing Pay Monthly customers can choose one of the three following options to upgrade to iPhone 3GS. I still have to understand why I would want an upgrade.... but that's for another day. At the moment all I know I do want an upgrade!
Option 1: purchase the iPhone 3GS on Pay & Go - well, if I had all this money to burn, I would have got a new phone already, thank you very much.... next....
Option 2: pay the remainder of your monthly line rental in one payment and sign a new Pay Monthly contract - ditto as above. this option includes the P word ('pay') therefore I no like.
Option 3: wait until eligible for an upgrade - I may already be eligible but if not, all O2 Pay Monthly customers are part of O2 Priority List and those paying £35 or more a month (and I do), have the opportunity to upgrade early. Customers won't have to see out the full term of their existing contract before being eligible for an upgrade. More in detail, all customers continually spending over a minimum of £50.00 a month, receive a 3 month early upgrade.
In other words, my next step is visiting an O2 shop to find out if I'm available for a FREE upgrade....