Can an antagonist change to helping the main character?

What a deep question for a Monday, a day devoted to reveling in youngest back at school and a completely quiet house. Yet, as I work through the first draft of my current YA, I find myself wondering if letting four characters my very alone MC thinks of as the pack become her supporters rather than her detractors is right - or is it right at this point. Okay, timing is everything, along with pacing, but it just plain feels right. Maybe I'll have some of the good, old buddy feelings back off as the middle progresses, taking it through to the sub-climax where my gal is forced into a do or die decision and then have those erstwhile friends realize their secrets can in no way be worse than the reason she made the wrong choice.

Hmmm? Deep thinking. Must mull this over with chocolate.