This would be really convenient; it's actually too convenient. Planning ahead and strategizing what minions get what and killing the useless ones off is part of the game. Because of this, I don't think an option to switch weapons will ever be implemented.
Issue with this is that you can't decide which minion will get killed.When you've got too many archers for instance, you're doomed as they are always the last ones to die.
you are not the first to suggest this, and you are probably not going to be the last
A middle way could be a posibility to "force give a tool" to a unit (maybe shown by a red arrow instad of a yellow one).while the cost of the old tool are considered as lost.
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