She's trying to fill Granny Aching's shoes but she is still learning. That's why she went to study with various witches. I think when she's ready she'll be considered a replacement for Granny Aching.

As Pratchett has said in another book, if Granny Weatherwax is replaced, it would most likely be with Nanny Ogg and Nanny Ogg's place would be filled by Magritt. Tiffany would fill Magritt's place.

It's been a while since I read that one so I may have the spellings wrong.