Note that when you are moving ships from one planet to another, all that matters is how far the destination star is from the nearest friendly colony. Your ships do not actually stop at one of your colonies to refuel. Also, when your scientists discover a new fuel type that increases your range, all of your ships immediately benefit from the discovery. You do not need to design new ships to use that fuel.

How do I change the destination of my fleets in space?
At the start of the game, once your ships have left orbit to go to another planet, their destination cannot be changed. If you change your mind during the same turn you gave orders, you can select the ship icon in orbit at the LEFT of the planet, and direct it to a new planet or back to its original planet. Once you develop Hyperspace Communications (34th level Computer Technology), you
can change the orders of any ships en route to other planets at any time.

I just signed a Non-Aggression Pact with another race, and he attacked my planet the very next turn. What gives?
Just like you, the other races cannot change the orders of their ships and transports en route. The attacking forces were probably sent to your planet before the pact was signed, and did not know that you are now on friendly terms.

I just got the technology for Controlled Barren (or Tundra, Inferno, etc.) Landings. Why can’t my colony ships land on those planets now?
Once you discover the technology for landing on a hostile planet type, you must design a new type of colony ship with the special base device for landing on that environment. However, that new type of colony transport can also land on any less hostile planet. For instance, a ship equipped to land on inferno planets can also colonize dead, tundra, barren, and standard planet types.

How do I conquer an enemy’s planet?
You do not need to send a colony ship. You just transport enough people to that planet to defeat the populace there and capture their colony. However, since missile bases and enemy ships in orbit can destroy transports before they land, we recommend that you send battle fleets to your target planet first, to eliminate any threat to your transports.