Hi @RussWWFC ,
According to the terms and conditions, it's not possible to have two Data Add-Ons simultaneously anyway so you should never be in the position of being double billed.
I do recall reading somewhere that adding a new one removes the old one but that may be outdated information, or something in the interface which has since been corrected to be more logical and user-friendly.
I see what you are aiming to do, you want the cost of the previous purchase to contribute instead to the cost of the more expensive purchase. I would recommend that you start an online chat to discuss this with the service provider before your first data add-on runs out, and they may be willing to help if you are persuasive.
The default method is that you run out first and then pay again. I think the Unlimited Data Pass would align with your allowance refresh so it would be better to buy that after the allowance refresh. If you have Reserve Data, you should probably try to hang on unless you are near the previous refresh and can benefit from unlimited data in the remaining time. Good luck!
And you can’t buy another add-on while you’re using one. So, there’ll be no unexpected bill
https://ee.co.uk/content/dam/ee-help/Termsandconditions/data-add-on-terms-jan-2021.pdf
https://ee.co.uk/help/help-new/billing-usage-and-top-up/add-ons/what-are-pay-monthly-add-ons
