Looks like the trick is to use MODCB to switch the RPL from UPD to NUP and back again. Here is the essential code I used and now it works fine. Non-essential code was omitted for sake of brevity, so ...
I have an Assembler program which reads a control file of record keys used to update a key-sequenced VSAM file. When a matching record is found, it must be updated. When a matching record is not fou ...