Originally Posted by SEA1K4EVR

No it definitely does not mean you'll have to give it up at checkin. If the assignment holds you won't have to pay for it. How the system usually works is that you have to provide a credit card number before selecting an E+ seat. But if you've already got the seat assignment then you've bypassed that step and you're golden (unless it gets taken away for some other reason in the mean time). Most likely you'll be fine unless there's a schedule/equipment change between now and the flight that gets you booted back.