I personally don't like the idea of doing that, I would probably just use a separate field, but the company I work for is asking about this..

You can change the label of a field on a specific form easy enough - if they require different field labels I'd assume a different transaction "type", and good chance there's other fields that should be shown/hidden/re-labelled? Could use another form for that if so - might be overkill for one field...

You want to use the scriptcontext.form.getField call, then once you have the field you may edit the label