Due to technical reasons, it is not possible. We performed a massive rearchitecture of Triple Town in order to support the new Capital City and also to support multiple cities at the same time (you may not have realized, but you can have multiple concurrent games now!). Forcing the game update after completion of the settlement is the only way to ensure your current town does not get glitched or broken.
Take all the time you need with your current town. Capital City isn't going anywhere :)