You still need to chang the DDS for the file.
But then use CHGPF. This will create a file according to the new specs, and will also keep any existing data. (As long as you do not change the field names)
You will however still need to recompile every program that uses this file and/or any logicals on this physical.
Make sure you take a backup copy before you do the CHGPF in case it doesn’t work correctly. Normally shouldn’t be any problem, but you never know….