Upgrading Heros

For giggles, I kept buying one of my heros a drink in the tavern every time he popped up. When the bar gets full, you can no longer upgrade that hero and that seems to be the end of the progression for that hero.

My suggestion would be to maybe make it a progression to where if your buying a common hero a drink, after you fill that bar, they progress to the next level.

Example.
White - Common Hero.
Fill the bar, and the hero becomes uncommon (green)..
Fill that bar, and the hero becomes Rare (Blue)..
Fill that bar, and the hero become Epic.. (Purple)
Fill that bar, and the hero becomes legendary. (Orange)

Maybe also make it so as you upgrade your heros, you can assign what skills they have (one time, no resets unless you fire the hero and start over)