It is possible to keep purchased names for a later time, not only possible but kept for all time, locked to your account. But sometimes one might want to buy a name for later use. Today if you want to do that, you need to first buy it, which changes your name immediately, and then pay to switch again back to the previous name. I'd like to request a tick box at the time of purchase labeled along the lines of "Don't change to the purchased name now".