I had the Explorer card. After 10 months, I applied for the Club card to get the first year free ($395). They asked me to move credit around, so I moved my Explorer's credit line to the Club card and closed the Explorer.

Is there some reason you need the Club card ASAP? You get two free United Club passes per year with the Explorer card. It was really the 1.5x spend that drew me to the Club card.

If you do not want another application/pull, then you will need to leave the Explorer card open at least a year before upgrading to the Club card, as others have mentioned.