When you delete a custom style that has been applied to content, that content changes its style to the "based on" style that the custom style had set. For example if MyStyle is based on "Body Text" and I delete MyStyle, then the content that used to have MyStyle applied to it now is using the Body Text style.

As far as I know, this is how all versions of Word work. You can't have content without a style applied so something has to happen. I presume that deleting a custom style that is NOT based on any style probably reverts the content to using 'Normal' but that is just a guess.

If you have outline levels changing then you need to look at the differences between the custom style and the 'based on' style. If one of the differences is outline list level, what do you expect would happen?

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Andrew Lockton
Chrysalis Design, Melbourne Australia