Unless you have a sofware package that builds a file for you, you have some work cut out for you.
You can use FNDSTRPDM over your source files and file all members that contain the value of the field name. Then you need to look at each one to see if you need to make any changs.
Also you need to file and recompile all programs that use the file you are changing where you use that field or not. Else you will get a level check.