I think even if you limit shape-changing to animal forms, you get a lot of utility out of it. I'd be wary of reducing the cost, but you could probably do it. But i would under no circumstances make it cheaper than -3.
That said: Imho "Some forms are harder to change into" is not a big enough disadvantage to justify a cost reduction in any case.
I'd handle it as a compel on his High Concept.
So if the difficulties the Character is having with changing to specific forms make his life complicated, then he gets a fate point.
If he changes into a really hard form in the safety of his own Home and then goes out he gets nothing for the difficulty.
If he wants to change mid conflict and it takes him several rounds... then he gets a fate point.
Or if he has to dance and sing to change into a form, but he doesn't want to get noticed, he gets a fate point for his trouble.
I think you get my meaning.
By the way: Whenever a shapechange requires a reallocation of form points via Modular abilities it takes a full action in any case.