Your contract or what ever messages written in the msmq as an XML object. If you add more property it will convert those to xml as well and write it to MSMQ.
Now, it depends on the process that picks up the messages and convert it to appropriate objects and do the processing.
So if you change the contract and dont update the service that picks up the messages from queue and do the processing it will fail. If you update the service with the same contract dll, it will still work fine, even though you dont necessarily have to use
the new property in processing
Unit test is part of TDD or agile development but also part of real world application development. Real world app development doesnt mean write something and put straight away into production. What i mean you need multiple test cases to meet your all situation
and make sure it PASSes every situation.