If this happens often, you can use the 'Set projects to project compatibility before building' option on the Make VB6 step to ensure any breaking changes will build without complaint.
If this happens infrequently, you might consider instituting a policy that developers must update and check in the individual VBP whose compatibility is being broken, set to project compatibility, along with the breaking source code change, and use the 'Set projects to binary compatibility after building' option in the Make VB6 action to ensure all projects get set back to binary compability after every build.
See this FAQ for additional options:
http://www.kinook.com/Forum/showthread.php?threadid=57