Legally, you're not bound if the location, shift, hours or department on the contract you signed, were not what you're going to be doing. Those details would make the original contract you signed null/void.
Ethically, it's not recommended that you change companies in the middle of a transaction. It is done though. If this is your first time dealing with the travel company, it is likely that they (and probably the facility) will not want to deal with you in the future if you now refuse to sign the second contract.
Your call. There are a lot of other facilities and companies out there.
Andrew Lopez, RN
http://www.4nursing.com