These are both costly solutions. I suppose the reason behind this migration  needs to be clarified. the problem statement is not clear enough.

If you want to provide a java-based interface to outside of the COBOL architecture, you can design an interface to the existing system. or as it was mentioned by Arash, you may be able to provide a CORBA interface with existing system, otherwise rewriting or rearchitecting are both long term solutions and you have to go through the whole process which will definitely duplication the testing effort of the components that are already written and responding fine (in COBOL). sometimes providing ONLY an interface (Web, J2EE-based, or any other interface) to the legacy system  is a good short term solution and satistfies the big  customer. JCA interface could be one simple solution and you can keep existing reliable part of your COBOL system and avoid duplicate and possible complicated testings.