It should work by you just phoning them up, they say they'll come in a couple of weeks in the morning, they fit the new meter and there's no charge as long as there's nothing to pay on the old system. You should use powergen, in my view, until the meter is fitted.
If you don't have a good payment record, they can ask for a deposit.
Sometimes the supplier refuses. Prepayment meters earn them more money and it's easier to collect it. If they refuse and you have a decent payment record, you can ask energywatch for help with your case.