It may depend upon which version of Word savefile you have, but because there isn't plain raw text in the save, but all kinds of formatting and markup hints, which is at least part-compressed but may leave some literal stuff 'open' anyway, your plaintext swap may have interfered with some formatting premise. Like "...and now we have 169 bytes of text, after which there's a new block of possible font-descriptors" which could go wrong if you s/Lucy Wu/Charles Henry Fotherington-Smythe, Jr./g (or vice-versa). Or it may hit the lazy compression algorithm, that declares plaintext bytes and refers back to them later (except now the reference is changed, data-munging.
Assuming the simplest answer wouldn't work (taking the Wingding document and reasserting that it's Arial/whatever, because only that changed and it's not a hotbed of now corrupted multi-font/multi-sized/multi-decorated text that needs more than a handful of global changes to somehow revert whatever you didn't mean to do with it), I think it might be best automated if you're using .DOCX format... Albeit that I personally avoid that for various reasons.
.DOCX and .XLSX, etc, are (from memory, can't check this) compressed by a common method (essentially .zip?) from a set of .XML resources. You may be able to extract the .docx into the component .XMLish file(s), box clever to make sure you only sed the necessary tag contents with your required substitution and repack the .XML into the .DOCX again.
If it's .DOC, it is a somewhat more proprietry format (though well studied) that you might need better file-object manipulation tools for (e.g. a Perl/Python/whatever module and then a very simple regexp between read/decode and encode/write stages, and that option also likely available for .DOCX if you're into Perl/Python/whatever already).
It depends how much time
you'll spend to save, which you might
not immediately know for sure and even thinking about that might
make it far worse for you. I'd be tempted to take the hit of a known "open with Word, replace textA with textB, resave and close" process if it's not expected to be a daily task, though a batch-conversion would always be
theoretically possible.