Currently the alternative is to stay on the current contract. However, I'm skeptical that will be the case as a few months before I joined, they had a similar situation; they offered employees 8 days extra holiday in return for working friday afternoons (used to do a half day on friday).
Half said yes, half said no. In the end as most had signed, those who hadn't were pretty much told to sign or leave. Although I'm not sure how accurate this is as I wasn't here at that time.
doubt half would agree to this though as it's a rubbish offer so that may make it more difficult