If this is a flat file with just one field, I beleive you need to do a new CRTPF with the new RCDLEN
If it has multiple fields, you need to change the DDS or SQL and create a new file.
Either way you will need to fo a CPYF to get the data to the new file.
If there are LFs, they will also need to be recreated.
All programs referencing this file will need to be recompiled.