ORIGINAL: AlvaroSousa
Easiest way is just to set all fleets to fleet mode if they merge with another.
Problem (and exploit) is that you can use this to move in one mode then switch it to another mode.
For example, if you use a unit to resupply a beach it automatically goes to Fleet mode making it vulnerable to attacks. By moving one last unit after that in Raider mode you can switch the stack back to Raider mode.
Other possible uses is to move in one mode to "see" what the situation is (like whether there are a lot of enemy around) and then switch its mode if it isn't the best one for the situation by moving a last unit in a new mode.