Since ESO was launched it's come in two editions, the normal and the imperial edition. They've always offered an upgrade to the imperial edition for those that want it. Nothing unusual there, pretty much all games do this stuff.
It turns out though that now, if you buy the account upgrade, it only works on the server you bought it on and not your account. So you have to buy it separately for each server. They don't tell you this when you buy it, the game lists it as an account upgrade that gives you "all the perks of the imperial edition", when it obviously doesn't.
There's a thread over here: http://forums.elderscrollsonline.com/en-GB/discussion/232691