Marketo API to "Update Email Content"

I've been trying to update the fromEmail and replyEmail for a large number of emails within our Marketo Marketing Activities, but even though I get a success response the two fields never get updated. Does anyone know if there's something I should be looking out for? I double-checked my API users permissions, I tried the quirky stuff like testing on a net new email vs a newly cloned email. Nothing appears to be working. I've successfully changed the names of the assets, but this was just as a test to see if it would work. I'm totally stumped on why these fields won't update. 

I'm referencing this doc that says it's possible to do this, https://developer.adobe.com/marketo-apis/api/asset/#operation/getEmailContentByIdUsingGET.