You could change:
.MatchWholeWord = True
to:
.MatchWholeWord = False
but this might have quite undesirable effects. For example, if the found word also forms part of longer, unrelated, words, those might get updated too. You might get better results inserting:
.MatchAllWordforms = True
after:
.Replacement.ClearFormatting
but that, too can have undesirable results. Test both on a document, with:
.Replacement.Highlight = True
inserted after:
.Replacement.ClearFormatting
and set your preferred highlight colour before running the macro. If you don't get the desired results, you'll need to insert an additional F/R expression for each plural.

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