1. I'm calling it Glinda the Good: she wears it everytime she's in the bubble gown. It's basically just shoulder-length ringlets. If you look at it closely, you can see there's a part all around the crown. She wears it with her Crown & Wand
2. She then changes to her Schoolgirl wig, which in almost every incarnation is longer, either completely straight or loose waves. The only exception seems to be Erin Mackey and very early Broadway.
The rule: (note: Megan Hilty's schoolgirl wig is significantly curlier than others (e.g., Annaleigh) but it's still a looser style than her Glinda the Good wig)
3. Finally is the one I'm calling the Politician, which she wears during the Thank Goodness scene and the Engagement Ball/I'm Not That Girl reprise. It's a curled bun. At the ball, her crown encircles the bun. There's also kind of a loop on the side. I don't really know how to describe this at all. If someone is great at doing hair, maybe they could post a tutorial?
Symbolism: this is all just my supposings here. Clearly the Glinda the Good Bubble Gown is the finalisation of who Glinda is. She has made her journey, been changed (for the better) and now stands firm in the conviction that she will be Oz's new leader. Her gown and hairstyle reinforce this ideal. Having her hair down (but still styled, and with a snazzy crown) brings out the "the Good", and makes her accessible to the people, clearly they all love her, but now she's (somewhat) showing them the real G(a)linda. She's come back to wanting to be personable to people, not just wanting them to put her on a pedastal.
Flashback to Shiz: her hair is younger, just like her. It's probably the latest fashion, and it's beautifully styled, and perfect for flirting with.
Politician: In Thank Goodness (and at the Ball) she's playing a part. She's dressing up like all the big girls, and she feels like she needs to wear the green everyone else in the city is wearing, put her hair up like a grown-up, and look very formal and sophisticated. And it's not until Fiyero leaves her that she reaches what we see in the beginning. Not just dressing like a leader, but becoming one to her core.
Anyway, this is what I do instead of writing my English paper for Critical Thinking. Hahaha.