how to make upgrades work

hi there, I was wondering how i cud make it so that a upgrade works for all units. Like im making a footmen wars map and each team has a unit(night elfs=sentry, orcs = grunt, humans = footmen, undead = ghouls) I know how to make it so you can re search the upgrade, but once it's done researching it doesnt affect them, so basically im wondering how to make it so the upgrade "steel weapons" can affect all the grunts, sentry, footmen, and ghouls.
Thanx in advanced!