Globally change - what ? JCL, COBOL Source? ASM SOURCE ? PANEL- SKELS - MSGS?
there are products (FILEAID,PDSMAN) for example that can change text (ab to xy) in all or some PDS members, but you need to take care, for example; if the source is JCL, the utility is non forgiving, it will change (ab to xyz) with no problems, and not care if the changed text now created an invalid jcl statement, or invalid source statement, I agree with Enrico, learning to write an edit macro and ensuring your changed data is correct is the best route.
ISPF LM - services along with an edit macro can do whatever you need to do to a PDS http://publib.boulder.ibm.com/infocente ... lmopen.htm