Q: Phone Unlocking vis Bill pay; will the phone automatically lock to a new network or am I free to use Sims?

If your carrier unlocks your phone it should work with any carrier, and you should be able to switch carriers at will. However, there is a halfway step called SIM Free. A SIM Free phone WILL lock to the first carrier it is used with. AFAIK carriers do not have the option of setting up a phone as SIM Free - that is normally only for stores that sell phones at a discount but represent multiple carriers.

After your contract obligations are met you carrier can unlock it for you usually for a fee. All it means is it's no longer locked to them and you can use it on any other compatible system. If you are changing carriers ask them about the terms and conditions.

As Far As I Know.

Meteor luckily do offer an unlocking service .You will need to contact then to understand the terms and any charge for unlocking your iPhone

Once correctly unlocked by Meteor it will remain open for any Apple approved carrier

read this to understand the process

http://support.apple.com/kb/HT5014?viewlocale=en_US

You should be able to use it on any compatible carrier you wish once unlocked. Again, contact your current carrier and future carrier for the details.