The answer depends where you purchased the device.
In the UK, all telesales & online purchases come with statutory 14days "change of mind" right of return - and it's impossible to inspect any goods without breaking a seal, so this is immaterial.
No such right of return exists for instore retail store purchases, and EE shops don't offer one. So if you bought a device from an EE shop, then you are at the mercy of the store's discretion which is not generally offered with upgrade phones as you have ample opportunity to be sure, before committing.
