Run a batch job that inputs the current MDC code (base code + today's date) and specifying the new MCD code (the new base you want + today's date).
//LIBR EXEC PGM=AFOLIBR,PARM='NRJS,NJTA'
//MASTER DD DSN=your.master.file,DISP=SHR
//SYSPRINT DD SYSOUT=*
//INDEX DD SYSOUT=*
//OSJOB DD DUMMY
//SYSIN DD *
If the Master file is part of a CCF Library Chain, you need to update CCF with the new MCD code as well.
See TEC1223567 for instructions on how to update the Library Definitions in CCF.