I had to think about this question before I posted, simply for the fact I couldn't make up my mind right away about the changes in Maggie, because there were definitely changes, but they happened quite gradually IMO.
It wasn't as though she became the childrens' governess then just as quickly snapped in to the "helpless" mode".
She had a steeliness to her that Vicki never possessed, a no nonsense attitude, which helped her to put up a good fight with Quentin's ghost when he wanted David.
she could also be kind and gentle with the children.
As we see her after the 1897 story line, her character transforms somewhat, but not entirely, in to a carbon copy of Vicki, although there is still that small spark of fire in her when she deceives Jeb.
When she takes the most drastic change IMHO, is when she is the victim of Roxanne Drew, and I don't think they gave her character a good sendoff by taking her to Windcliff.
Did she change? No doubt.
Do I miss the strong, slightly rebellious Maggie? Most definitely!
I wish they would have allowed her to remain her own character instead of trying to make her in to another Vicki. I liked her much much better as Maggie the working class waitress, friend of Vicki, and fiance to Joe. What a pity that all ended.