Each move allows moving by up to two fields (the round spaces on the game board). If the ship is empty (there is currently no creature on board) during the entire move, it is possible to move by up to three fields.
If you start with an empty ship, move a field, embark some creatures, move another field, disembark those same creatures, can you move another field (your 3rd) with your now-empty spaceship? I assume not and that to be able to move 3 fields, your ship has to be empty the entire journey.
It works like this: you remove all your tokens of the mutated race and replace them with the same number of tokens of the race you are
changing the creatures to. The good news is that within this action you can place the new tokens on any free space in the grid. If there is not enough space for all the new creatures, you can change the lower number of creatures so that they fit on the planet.
Suppose the mutation relationship is Toadlings Sluggies. If I have 4 Toadlings and 3 Sluggies on a planet, and I mutate my Toadlings to Sluggies, I'll run out of Sluggie tokens. What do I do? Do I place the Sluggies I can and ignore the rest?
Suppose I have 3 Toadlings and 1 Sluggie on a planet. If I mutate my Toadlings into Sluggies, remove all my Toadling tokens, and realize I cannot fit 3 more Slugglies on to the planet, what do I do? Do I place the Sluggies I can and ignore the rest? Do I return the Toadlings (hopefully remembering exactly where they used to be) and then mutate my 1 Sluggie into a Toadling?