Function which edit other functions in PostgreSQL

For example, I want to rename some function and all it's calls from other functions. I would write the function rename_func(schema_name text, old_func_name text, new_func_name text), which must edit other functions.

Where does PostgreSQL store the body of functions (using plpgsql)? I cann't find such column in system tables.

I just can get all functions with

SELECT f.proname, f.prosrc, f.probin   -- trouble: f.prosrc and f.probin are empty
FROM pg_proc f;

but I cann't access to their code.