You assign models to markers before the first turn, during deployment. You swarm at the start of an action. We've always read it that units starting underground needed to have a marker each. It's the whole point of having tunneling assets.
This opens the rules up to abuse..... why buy warriors in 15's? Because in fives they take lots more reaction fire. What's the negative? It's harder to bring them up above ground due to the ten size points an action limit.
So you have to weigh up the advantages of unit size vs manoeuvreability and buy units accordingly.
Not any more though, just buy them in fives, swarm before the game and you have the best of both worlds. Units of five acting as units of fifteen, until that's not convenient, then they split.
That removes a major strategic decision from a bug player (like myself) I'll just have three, five bug units swarmed under my markers.
And what about hoppers? Stick 15 hoppers (three units of five) with a burrower. That's a nightmare. Previously this was tricky as putting more than five in a unit made them hard to bring up.
Plasmas? Buy three, stick them with a burrower, pop up artillery.
And you claim that swarming tankers limits their movement? What about coordinate, you can now race all three up the board at 15" a turn can't you.
My new front line is three tankers central, flanked by a swarm of 15 subterranean hopper/burrowers and topped off by two plasmas underground at the back.
I prefer to play bugs, being allowed to swarm before the game starts makes things a whole lot easier, also, the MI guessing game of 'what's under the markers?' is now impossible, if each one could be a swarm, there's no point even trying.
I agree with being able to swarm underground, but not being able to swarm before the game begins.