Mainframe PROC UPLOAD ERROR

[Dawn3]

Please how to update mainframe SAS dataset that is part of PDS (partitioned data set)?
Using PROC DOWNLOAD the content of the MF member gets downloaded to LAN. After another SAS process updates this dataset (.sas7bdat file) I need to upload the new content from LAN overwriting the original MF dataset. Using this method produced authorization error:
. . .
signon . . .;

RSUBMIT;
LIBNAME MFDAT 'AAA.BBB.CCC.SASDATA.A0100 ' DISP=SHR;

PROC UPLOAD DATA=TEST.SOURCE OUT=MFDAT.TARGET;
RUN;
LIBNAME MFDAT CLEAR;
ENDRSUBMIT;

Note that I do have proper authorization and the same statement above with:
LIBNAME MFDAT 'AAA.BBB.CCC.SASDATA.B0100 ' DISP=NEW;
does create new B0100 dataset. When I PROC Download it, change the content, and try to PROC Upload it with:

LIBNAME MFDAT 'AAA.BBB.CCC.SASDATA.B0100 ' DISP=SHR;
PROC UPLOAD DATA=TEST.SOURCE OUT=MFDAT.TARGET;

I receive the authorization error.

ERROR: Write access to member AAA.BBB.CCC.SASDATA.B0100 is denied.

The datasets are relatively small so space is not a problem.

The environment is z/OS, SAS 9.4 EG 5.1

Thanks a lot

[SASKiwi · Accepted solution]

DISP=SHR sets the LIBNAME to read-only. Try DISP=OLD to write to an existing file.

[Dawn3]

Thanks SASKiwi! Yes, it works!.

[SASKiwi]

@Dawn3  - great to know. Haven't touched a mainframe in over 15 years but its surprising what you remember...