I don't know about automated per se, but yes, you should be able to do it a little bit easier from within Sigil.
You can add new font files to your epub by using File->Add->Existing Files (or right clicking on a folder in Book View and selecting "Add Existing Files" from the context menu).
You can then manually change the CSS font-face declarations to use your new font files and then delete the old fonts (right click on them in Book View).
Saves you the trouble of unzipping/rezipping anyway.
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