CICS still updates the VSAM file the new service also writes

We peeled off one process, but CICS still updates the same KSDS. Can the new service write it too?

Not safely, unless the file is under record-level sharing or transactional VSAM. SHAREOPTIONS alone are not enough when CICS is also updating the KSDS. Treat concurrent writes from the gateway and from CICS as unsafe. Let one side write. Use dataset CDC if the other side needs to hear the changes. CDC publishes events. It does not make two writers safe.