A bit late to the party, anyway, it's actually very easy to transfer your number from your existing contract onto a new Vodafone contract taken out through a third party. Due to Vodafone's 'troubles' with porting numbers and after AnnS helpful information on how to do it, I chose the first option of the two that Lee listed earlier on.
Put Vodafone on notice to change you to a PAYG which will take thirty days. You will get a letter through the post to confirm this with the date it will happen. To minimise the overlap of both contracts wait two to three weeks and then order your new phone. On the day your old contract becomes PAYG fill out this form, you will need your new account number (text ACCOUNT with your postcode to 97886 from your new phone) and your old SIM card no.
https://vodafoneinlifeporting.co.uk/ilp/web-forms/pay-as-you-go-to-pay-monthly/
Depending on when you fill the form out it can take one to two days (not weekends) to complete. Wait one to two days, when your number on your old phone becomes dead turn off your new phone then turn it on again, (a reboot doesn't always work) and your all set with your old number on your new phone.
Just to clarify a couple of the points Lee made.
You will not get a new SIM card, your original SIM card remains active but becomes a PAYG. There is no need to top up the PAYG SIM card before transferring your number to the new contract. That may change if you left it for a period of time but, certainly in my case two numbers transferred fine without any top up.
Laid out it looks like a bit of a faff and compared to just upgrading through Vodafone, it is. However it's not actually that much work as long as you get the timing right and as Vodafone don't suppy the phone you want, other than buying the phone SIM free which is what I would recommend, it's one of the few options available.
