I don’t actually believe that Epicor lets you change it.
What I’ve done in the past for items ordered early is use planning contract. These allow you to “allocate” (not in the strict Epicor terms) items to a contract, then connect the PO and the Job to the same planning contract. They are loosely linked then. Check out this thread. If you are used to buying direct to the job (as in you don’t need to see them in inventory at specific bin locations) it could work really well for you.
Expanding on this, sort of off topic - but Job related -
Once I schedule a job, can I make edits to it after the fact? Or does everything have to be complete as far as materials, operations, etc before scheduling?
You can change them. There are some settings on allow setting when engineered etc. But as long as there aren’t transactions on whatever is it you are changing, you can make changes.