It depends on what the procedure does.
For example, if it (along with returning some value) uses DML operations (e.g. inserts rows into some table), then function can't do that - you'll have to use a procedure.
Procedure's drawback is that you can't use it in a SELECT
statement, such as
select proc_name(out_param) from dual;
You'll have to use a function in such cases.
Also, OUT
parameter has to be stored somewhere, and that's usually a locally declared variable, but that means that you need another PL/SQL block to call the procedure and see its result.
If everything your current procedure does is to find & return some value, then yes - a function might be a better choice.