A contract does not end, it continues on a rolling contract until you give notice to end it.
To leave there are a few options;
Give notice you intend to finish the contract, this will take thirty days, after which your account will be closed, but you will lose your number this way which is not what you want.
To keep your number and transfer it to a new network ask for a PAC code within the last thirty days of your contract. This is valid for thirty days. Take out a new contract with another provider prior to the end date of your Vodafone contract and give them the PAC code on or around the end of your minimum term with Vodafone. It should take a couple of days to transfer depending on when you give the PAC code and they don't transfer over weekends.
This will terminate your contract with Vodafone on the transfer date. If your transfer goes through before the minimum term date, you will still be liable to pay up to that date.
There is another option if you wish to stay with Vodafone but have an HTC phone and that is to buy a phone from a third party supplier. Some like CPW can provide an upgrade but you need to be specific that you want an upgrade and not a new contract.
Or if that's not competetive enough you can take out a new contract with Vodafone through a third party supplier, put Vodafone on notice to change your current contract to a PAYG which will take thirty days. After thirty days when it's a PAYG you can transfer your old Vodafone number to your new Vodafone contract.
There are lots of ways to do it, timing is the key to keep it as close to your end date as possible.
