In my last game I got a bit ahead of myself and had a colony ship that did not have the range to get to the next available habitable planet. To stop it coming up every turn for action I set it to Sentry. When my technology had caught up, I could not change the Colony ship from Sentry to anything else, including moving to a Planet. The ship summery showed a destination, but alas the ship stayed right where it was and displayed Sentry as its current orders.
