Morphing into different things at different levels

I am creating a spell called "Bear Form" where a druid can transform into 3 different types of bears at different levels of the spell.

Using "Storm Crow Form" (Druid of the Claw) as my base spell, I made it so that it's a hero spell with 3 levels. Level 1 - Small bear, level 2 - Grizzly Bear, Level 3 - Rampaging Bear. I create 3 bear units and then set the Alternative form types to the 3 bear units.

However, no matter what level of the spell I cast, my hero ALWAYS transform into the level 1 bear (small bear). Is there a workaround or is there something I missed?

Use triggers to place special effects and then replace units.

So er... how do you do that? :p