Two cheesy exploits regarding ship upgrades.
1) Upgrading a constructor (that has used 4 out of its 5 construction points) to another constructor will give it the original 5 construction points for like 40 credits. Total cheese.
2) Upgrading a cheap constructor to a colony ship gives you free population to put on the next colony. You are effectively buying population for like 40 credits. (Amplified by having Benevelonce III giving you free additional 2 population when colonizing a planet, and heaven forbid you have the race trait giving you your first building free on said colony.) I build a cheap constructor, on first turn move it its max moves towards my target planet and then upgrade it so that next turn I have a colony ship heading to the planet with no drain on my building planet. Total cheese.