You can convert your file to be hold variable length records. The ones that are already 80 bytes will stay at 80 and any new records will be whatever length they are. But, as Dick intimated, any downstream programs/JCL will have to be changed.
You may be able to do the conversion by utility but why not write a program to do it and thus get practice in reading/writing variable length records?
The problem I have is that people can explain things quickly but I can only comprehend slowly.