19-01-2021 16:05 - edited 19-01-2021 16:06
From past experience, it will change from when the new device is despatched... so your next bill for the airtime will be slightly higher as it will have say for example 2 weeks of the extra on the upgraded contract (pro rata) and then your month in advance.
on 19-01-2021 16:10
Your billing date always remains the same and will be adjusted to take into account any price difference between your old contract and the upgrade.
OnePlus 6 (O2 & Sfr), Z3 Tablet (Three UK), iPhone 8+ (EE)