The option was avoided intensionally because players will choose to put the whole skill under attack or defense which will completely imbalance the system... making the cost really high can't solve this.
Also, conceptually you can't change the skill of your army once its already trained/deployed...
Pathetic idea, just reset like the rest that want to fix mistakes. This would be exploited and the Devs have already stated they won't implement this.
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If you're naturally kind, you attract a lot of people you don't like.
I agree with Michael. It would mean more income for Devs pocketbook if a one time do-over was granted for a specified amount of diamonds, say 100. Then players wouldn't have to reset. It sounds like good customer service to me. And on the topic of "making the wrong choice" how are newcomers to the game supposed to know how to make a right choice straight out of the gate?
Can't be implemented as it would imbalance the game irreversibly if a very high level player use to it put everything in attack or defence.