had this email from o2 this morning, I'd asked them (before the existing customer announcement) if they knew when id be able to upgrade, if it was online or instore and if they were doing a pre-orders.
Hello Richard
Thanks for writing to us about upgrading to iPhone 4.
Richard, I understand that you want to know how and when you could
upgrade to the new fabulous iPhone 4. I'll surely provide you the
details.
You'll be glad to know that you can upgrade from launch (24 June 2010)
by contacting your nearest O2 store or Online.
I would suggest that you upgrade by going to one of our stores on 24
June 2010. The handsets will be available on a first come/first served
basis.
You will be unable to pre-order as we only have a limited amount of
stock from Apple.
You can register online at http://shop.o2.co.uk/new-iphone/ to be the
first to know and receive regular email updates. We've had thousands of
customers sign up already.
I hope you find the information useful.
That goes against what they officially released yesterday, according to the link, iPhone 4s will only be available in store from the 24th, and not online until the end of July:
http://shop.o2.co.uk/new-iphone/wheretobuy.html
**edit** you probably new this already judging by your "before the existing customer announcement" comment!