Joined: 14 Mar 2007 Posts: 8657 Location: Back in jolly old England
There should be an ISAM>VSAM interface already defined on your system, which allows you to use VSAM files instead of ISAM files transparently to your application. Although, I last used this in the 1980's.
This allows you to continue running the same programs using VSAM instead of ISAM while the program code is changed and tested.
You can try to define a VSAM cluster and use this in your program, and if it works OK then the interface still exists, if not, then you will need to draw up a detailed plan of every ISAM file and which programs use it, and then start the controlled conversion process.