Can’t Change Default History Doctrine Class

In our ward, we had to split off a smaller gospel doctrine class for new members as we had a large influx of them. We created a new class by splitting the main gospel doctrine class and moved people into it. However, that’s had the unintended consequence of creating a second default class for all adults and now all new adult members moving into the Ward are transferred into that class.

Is there a way to remove that default setting from the second gospel doctrine class?

You can't control which one is the default class, but now that you know you could change which class is the main adult class and which one is your new member one so that the defaults go where you want them to.