Slitherine

is it possible for a unit of entire light infantry (French light infantry) to change formation from tactical to skirmish.
If yes, do we need a CMT

Yes.
Sometimes, if you are in close range of the enemy - see CM Table p 45 4th box.

Skirmish formation is good to reduce your losses in medium range firefights, but note that it takes 2 moves to form back to tactical and then to square if cavalry approach in the open. Evasion is the usual response.

MikeK wrote:Evasion is the usual response.

Or destruction as the case may be.