Sorry to hi-jack but need some advice. Been a while since I jumped phone companies.

Currently on O2 and want to switch to vodafone for the iPhone. Out of contract - rolling on every month.
Want to get an iPhone for Vodafone as it has better coverage.

I know I need a port number to give to Vodafone but when do I do this? Want to get the iPhone 4S probably from the apple store when it comes out, which is not something I've done before.
Do I get the port number the day before I go to the apple store?
Can I change numbers after I've bought the iphone + vodafone sim?

You need to get the PAC code from 02. Ring them and say you want to leave and keep your number. You get the code and then give it to Vodaphone when you sign up. Not sure how long it takes to get the PAC code

And presumably you just asked O2 to send you an iPhone SIM but staying on that contract?

It is the to get a PAC number before you get a new phone so you can tell it to Vodafone when you are getting a contract. It is also possible to get a contract from Vodafone first and then get a PAC number and give it to Vodafone afterwards.

From what I can remember, in only takes a few hours from calling Vodafone and asking them for PAC number (never done it with O2) until they send it to you in email. What is important though, PAC number is valid only for 7 days before it expires. If you don't use it in a week you will need to get a new one.

I guess what you should do depends on one thing:
a) if you expect to walk into the vodafone shop and come out with the phone, get the PAC before going to vodafone
b) if you you think they won't have the phone for there and then for you to take home, get your contract from vodafone, wait for you phone and then call O2 for PAC. When you get your PAC call Vodafone and give them to port you number.

My dad went through hell trying to get his PAC code from O2. They insisted on texting it to him, but obviously, he couldn't receive the text.

I changed to 02 and to get my pac code it took just a few minutes