Word uses the 'Footnote Reference' Style for the footnote references in both the body and the footnotes themselves. Accordingly, just changing the Style definition won't do. What you could do, though, is:
1. Open the footnote range (i.e. click on any footnote)
2. Press Ctrl-A to select all footnotes
3. Use Find/Replace to find all content in the 'Footnote Reference' Style and make it bold, not superscripted. Answer 'no' to the do you want to continue' prompt.

__________________
Cheers,
Paul Edstein
[Fmr MS MVP - Word]