I would have thought you could search for a ^p that is hidden but that doesn't work. Typically a style separator is followed by a space and it would be poor practice if regular paragraphs began with a space so you could use that quirk to search on.
Find ".(^13) "
Replace "\1. "
Use wildcards on

Failing that, is there some other consistent attribute you could search for eg all style separators are Heading 2

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Andrew Lockton
Chrysalis Design, Melbourne Australia