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Bravefoot Afternoon
I'm not totally aufait with prepayment procedures but what I can gather you should be using the key that Eon Next sent you. I found this statement whilst researching:
Traditional prepayment meters are programmed by the key or card that you use to top up. This means that if you top up using a key or card from a previous supplier, you'll still be paying their unit rates and standing charges.
The following information was from:
https://www.eonnext.com/help/prepayment-meters
I've moved from another energy supplier, will my prepayment card or key continue to work?
Both electricity and gas card or keys will continue to work while your account moves over. When your move is complete, we’ll send you a new gas card which you can start using straight away. If you have an electricity prepayment meter we don’t need to send you a new key and you can carry on using your existing one.
If you haven't received your new gas card a couple of days after we've moved your account, please get in touch.
There's no harm contacting Customer Services to check, but I would suggest using the key sent to you by Eon Next.
Ways to contact them in the purple section at the bottom of this page.