The creator of HalloweenTowen Sheri Singer and KJB sat down and did an interview together and reading through the lines it seems like they always wanted a blonde actress and jumped at the chance of a recast:

Singer: She came in and she wasn't who we had visually pictured, but she was the role. She blew everyone else away. She was great.

Brown: It was so exciting when I found out I got it because I remember loving the script and loving the idea of playing a teenage witch. And then hearing Debbie Reynolds was signed onto it, it was like, oh my gosh, I'm going to play Debbie Reynolds' granddaughter!

Singer: Debbie was coming in with the blonde look and Judith [Hoag] was reddish blonde, so I thought it would maybe be a lighter girl. Of course, this is so politically incorrect today, but they were all related…I had just envisioned a Blondie. But she came in and she was it.

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About the recast:

Singer: It was not something we wanted to do. We could not come to terms that we felt were fair. We just weren't able to. We couldn't make the deal work. That was why and we didn't want to not do it. I know people didn't like it, but it's not like people haven't been recast before. I always was sorry. That's how it went.

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enews interview source