About 20 days ago my wife bought a new phone and came with a scratch off code to redeem effectively a mystery bonus.

This turned out to be a free $55/month monthly plan switch for 1 month. It didn't offer this to her upon entry of the code, it immediately applied it. She's let me know that her minutes no longer show in the app - as she had quite a few carried over but because on the $55/month plan they are unlimited it's now not showing a balance. 

Does anyone know if this balance will be reinstated when she reverts to her $19/month prepay?

If she reverts now (10 days before expiry) will it come in to force now or at expiry?

Attempting to cancel the rollover on $55/month now in the app says this cannot be done via the app and asks her to call 2degress (which I've encouraged her to do since she's the customer not me). This has shades of the thread about NZ Herald not allowing subscription cancellations online...

While it was great she got a freeby with the phone, it appears if she didn't cancel she would be automatically charged $55.00 for the next month as that's the plan it's placed her on. This would be fair enough if she had chosen a $55/month one-off deal then forgotten to cancel, where this is different is what the code was offering was a total unknown and once she entered the promo code she was placed immediately on the plan which had potential pitfalls if someone forgot to cancel and couldn't afford an unintended charge the next month. A one off data/minutes pack would have given the same value without the risk.

I get that this was probably designed as a good thing, but changing someone's plan to a more expensive one without giving them some form of heads up is a slightly backhanded promotion.