It sounds like it would work, in theory. But I don't know enough about CC-making to guess whether changing the GUID in one place is enough to create a viable new piece of CC -- with respect to the original "good" eye, since you said you're making a copy and not introducing a new eye file. There may be lots of other little values that need to be tweaked, otherwise you would have two almost-duplicate files. (Unless you ARE downloading a new file...) For one thing, I don't think "the" GUID of the file itself, that distinguishes it from the contents of all other .package files, has anything to do with the "family" property in the eye XML -- the value that is used for assigning the eye to Sims, in other words. So that's two values there.

But keep in mind I am no expert in modifying eye files. I just change my Sims one-by-one.