I have two Apple Watch Series 3 Cellular on my account (one belonging to myself and the other belonging to my wife), and have now purchased (directly from Apple) 2 x Apple Watch Series 7 Cellular's to replace the above.
Ahead of this purchase I contacted customer support via chat about transferring the current data plans from the Series 3's to the Series 7's and was advised that this wasn't possible and that I'd need to cancel the original plans and would need to take out new plans for the two new watches. The agent in question then proceeded to serve 30 days notice on both current watch tariff's.
Low an behold the new tariff's are coming up more expensive than my current tariff's and based on this thread others have seemingly managed to get their plan's transferred to their new watches without issue https://community.ee.co.uk/t5/Apple-Devices/Transfer-data-plan-to-new-Series-7-Apple-Watch/m-p/10885... Both watches and associated iPhones are all on Pay Monthly plans.
So who's right?
