Rule reminder
'A meno che non' + subjunctive = exceptive clause (unless). 'Se non' + indicative/conditional = conditional negative (if not). Both translate roughly as 'unless' in English, but 'a meno che non' is more formal and always takes the subjunctive, while 'se non' takes the indicative or conditional and is more colloquial. Example: 'Non partirò, a meno che non finisca il lavoro' = 'Non partirò, se non finisco il lavoro'.
In formal written Italian, which is the preferred way to express 'I won't sign unless they change the terms'?