I don't think it's worth it.
Disclaimer: Started career in COBOL on mainframes. Hold Sun Certified Java Architect and Java Programmer certs. Have worked in professional Open Source (mostly Java middleware) for over a decade.)
I'm not sure the Java will be more readable. Java's dependency/library spiderweb is much more complex and fragile. COBOL on the mainframe is maintained from a central position. COBOL won't have the attack surface Java will (see previous point about dependencies).
I think it might be better to just train newbies on COBOL.
Caveat: Maybe cloud economics can make a compelling budgetary argument.