Most of the time you use combos to choose list items and then do things like filter records or choose list values to save in a new record, thus you do not bind them to anything (do not use the combo controlsource property). If you bind them you will set whatever field you've bound it to become the new chosen value. Forms with unbound controls usually need code to save or edit a record using the unbound data.
If you need to bind in your situation, then you must either realize the result of choosing a different value, or you need to use a single form view and disable them for existing records and enable them for new records or if you can, swap form mode from view to edit and disable/enable accordingly.
HTH
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.