You can change thier gear fine.
In the schedules menu, select that squad and set them inactive=civ clothes, I think the hotkey is u, do this for each month.
Set the squad to inactive.
wait. I mean it, almost every time I see someone complaining about gear it is because they expect thing to happen instantly, doesn't work like that.
Define a new uniform and assign it to them, you may want to use the "use exact matches" option if it is too similar to their previous gear.
Switch back the inactive option in the schedules menu, then set the squad to active/training. (If any of their gear is inside of a minecart it will completely screw up pathing, make sure that is not the case first.)
Make sure they have somewhere to train and then wait. They heard you, they'll pick up their gear when they bloody well feel like it, yelling at 'em wont make it go any faster.