Just a thought, but rather than a 'condition unchanged' link, randomly pick one of your review metrics and give the user a single click option to rate on that instead.
Another option might be to invite them to reply to the email with any follow-up thoughts on the product to add to their review.
I'll be happy to help you if it's two minutes once a year, and I think that's all the info you need.
"...rating by default is the equivalent of saying "everything is okay". That rating is made without you having to lift a finger. You only have to lift a finger to change it to "awesome" or to "sucks" or to add a elaboration for others to read."
Also, the final section of DefaultReviews [1] may also help solve your inevitable problem with faked reviews. Pull requests welcome.
[1] https://gist.github.com/iL3D/59df64947d42828d848ebfc1651a312...