Depending how intent they are on pushing this through you may want to be careful how strongly you word your objections.
For instance, if you were to use as your template something like the one included in Discovering's recent post a very likely response could be:
"We regret you do not agree to your contract revision. We will consider your contract to be cancelled effective June 30, 2009 unless we receive from you written agreement to the new terms within 48 hours".
If that sounds overly dramatic I have personally observed the case where a certain Captain wished to resign from a stressful Flight Ops management position in order to return to the line. The official response he received was that his resignation was declined but should he wish to insist upon resignation he should be aware that there were no line Captain positions available for the forseeable future.
Hardball is as hardball does.