[wulfir]

Skirmisher,

buy a sniper and assign all units you want deleted to this formation go back into the buy screen and then delete the entire formation. Myself, I don't like using this method because if you don't watch out the unit flags of the purchased units you delete will be left on map (there's a workaround for this problem, basically you gotta buy more units equal or more to the number deleted if you want to avoid the "ghost unit flags").

IMHO the smart route to take is to buy a singular infantry unit, a sniper or maybe a scout and use the change unit function and make it an engineer squad. Easy and quick.
If you are feeling ambitious you can of course edit the morale/exp/leadership ratings of the new engineer squad, but I don't think it's neccessary if the converted units originally was an infantry unit.