Yes you do have to pay more if the other seat costs more.. prior to the merger UA charged a flat fee to move into E+ because it's system couldn't handle dynamic pricing. But now it can..and so they charge different prices depending on the seat. On board they're not sticklers about that though so if the flight ends up not being very full you can move around within E+ (they won't let people move up from regular economy but if you're already in E+ you can do it). Not that it's likely to be empty. If you really want it I'd pay for it.