You can also change ships between planets by sending on a recycling mission, so the planet you want them to end up on send those recyclers first, then add with the planet you want to reduce your fleet from they will all return to the 2nd planet that the first recycling fleet came from. I discovered this by accident. You want planet A ships to end up at planet B then send planet B first once they are in orbit recycling send planet A ships they will all go to planet B once recycling is complete.