Without seeing the code there is no way of knowing the scope of this change.
Contral break processing and O-spec output control might be linked directly to the primary and secondary files P/S specs.
Primary file will need to be read in the main processing loop .. which probably didn’t exist in the previous code.  Secondary files will need to be chained to at the appropriate times.
To make this change you must understand how the program currently works.

A ‘P’rimary file will generally be replaced by a straight READ-loop around the main calculation specs. A ‘S’econdary file, however, is going to require some thought.
 
Secondary file processing can also require READ-loops. There are often more secondary records for a given matching key value than there are primary records for the key. And there is consideration needed for missing primaries and missing secondaries, and for EOF against either file.
 
As Phil mentions, issues involving control breaks (‘level’ breaks) and O-spec output also must be considered. Output is likely going to need to be significantly restructured.
 
Overall, unless there is a strong reason for making the change, I would recommend against it. It should not be done if the only reason is to change the file processing modes. It has a reasonable chance of needing essentially a full rewrite of the program. In fact, I would start from that assumption.
 
Tom