As NicC has pointed out, you are running on a PC. This is an IBM mainframe Cobol forum. There are sufficient differences between mainframes and PCs for us to give you the wrong advice, so try looking for a forum for your particular Cobol compiler or at least for your operating system.
A general point about a record-layout. If you change a field's length, and there is already data after that field, then you have to "convert" the file so that the data conforms to the new record-layout. You should (on a mainframe, anyway) reduce the length of the filler by one.
The "cheap" way to do it is to change your existing PERCENT to FILLER and then take four bytes from the FILLER at the end of the record, leaving the remaining filler as the last field, and put your four bytes percent and redefinition there. If it is a system for yourself, you can do it that way, if it is a system where you are on a team, find out from the leader of the team, who is responsible for changing the record, converting the file if necessary.
Good luck in your new forum.