They can't do it without your consent.
They must ask for volunteers at boarrding time, what you should have done is show up at the airport anyway for your original flight.
I say don't accept any changes that the person calling on the phone wants to do.
Then after finishing that phone call, call or log onto the airline and find out whether the flights were cancelled.
If the flights were cancelled, then rebook.
Travel tips:
http://members.aol.com/ajaynejr/travel.htm
Alternatively you can ask immediately what the compensation for being bumped will be. This is tricky because there is no way to give you your voucher over the phone and also you can[t get written confirmation of what was agreed upon over the phone. Better to actually go to the airport and volunteer if that is what you want to do.
[This message has been edited by AllanJ (edited Jan 01, 2004).]