Read this before complaining me.
So we will have all classic skins inside one skin. I mean, when you click on the miner skin, we will get a tab which shows more upgradeable skins. It only can be bought. Or you can get it from chest. Other skins such as Leaf, voltaic won't get this change. Only units with classic skins get a upgrade.
Here's a example:
Miner classic skin- Free.
Miner classic first skin- Costs.
Miner classic second skin- costs.
Miner classic third or max skin- costs.
I hope you understood. Not only miners but also others units will get this.

