Update a field definition without loosing form data

[Mick]

I have a bunch of MS Word forms (about 200) made from a template with
several fields (text, checkbox, and dropdown). I need to update some of
the dropdown fields to include additional values.

If I unlock the form, add the new value, then lock the form, all of the
data in the text fields reverts back to the default text.

Anybody know of a way that I can update my dropdown fields without
losing my data?

Thanks,
Mick