It appears to me that those images are from different files. Not only are some characters different, but so too is the page layout.

Word has not changed µ to m in the 1st paragraph. Nor would it change Λ to D in the 3rd paragraph. The only thing that could account for the character changes is if you previously used a non-standard font with non-standard character mapping for the symbols and haven't installed that font on your new system. In that case, you should update the file by replacing those symbols with the correct characters in a standard font. They're included in Times New Roman, for example, in the Greek character sub-set (see under Insert|Symbol). This functionality has been available in Word for many years.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]