If you mean to change the Resistance man's weapon, then you need to remove it first. This can all be done in a script, but it's easier to do it in the Initialization field of the soldier, just like you said.
Unitname RemoveWeapon "
WeaponName"
Where UnitName is the name of the resistance soldier, and WeaponName is the name of the weapon (suprise surprise)
You'll also need to remove the magazines.
UnitName RemoveMagazines "
WeaponName"
Afterward, add the weapon you want the Soldier to have, but put it's ammo in first, to make sure that the weapon is loaded.
UnitName AddMagazine "
WeaponType"
The Weapon next:
UnitName AddWeapon "
WeaponType"
Be sure to put in the
; characters inbetween each line of code.
Hope this helps!