Normally you would have to pay off your balance before upgrading @Beck3 , unless you are on a Switch Up tariff ( Guide: Introducing O2 Switch Up. Swap phones whenever you want! ).
If you take your phone in to your local store they may be able to say roughly how much you would get from o2 recycle, but your phone would have to be sent off to be valued. And they sometimes play fast and loose with valuations...
You could alternatively visit a CEX or similar and ask them, but again you would need to pay off the balance before upgrading.
Alternatively you could check
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