Randomise spell

I think there could be a spell where you can put a troop inside and it can change the troop into a better troop
Every level you have more chance of a better teir and or level troop
Lv1
Cost: 5000 elixir?
Duration: 5 seconds
Improvement of troops: up to 2 tiers or 1 level

Upgrade cost 100000 elixir?
Lv2
Cost: 10000 elixir?
Duration: 7 seconds
Improvement of troops: up to 3 tiers or 2 levels

Upgrade cost 200000 elixir?
Lv3
Cost: 20000 elixir?
Duration: 10 seconds
Improvement of troops: up to 5 tiers or 3 levels

Sound like a decent idea, but need more info. What will the troop improve upon?