Making changes to the definition of a user-owned indexed set can be difficult.
This article contains an application program that can be helpful in this area.
IDMS: All supported releases.
Download the *.zip file attached below and unzip it.
FTP the file kd219347.bin to your mainframe in BINARY mode to a dataset with LRECL=1024 and unpack it with TRSMAIN. The resulting file is a PDS. Read the README member.
The members of that PDS are also provided as *.TXT files when you unzip the downloaded file. The content is the same as what is in the PDS and are provided as .TXT files for convenience.