Thanks, all.
@DiapDealer - I'd love to have that plugin (but have no idea how to make my own).
I think I'll try the UUID-generator site. In the case where this came up, I did try a bunch of things to make Sigil generate a new UUID, but they didn't work (perhaps because I don't have obfuscated fonts). I messed with the opf, but that seemed to make things worse. I finally just transferred all the files to a new EPUB, but reordering a bunch of files is tedious.
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