I said it before, and I'll say it again: any ticket subject to English common-law rules would typically fall under the rule of unilateral mistake. In this case, Alitalia's only remedy would be rescission, that is, voiding the contract. Alitalia, very stupidly, decided to rely on self-help, and unilaterally altered the contract instead of using the only remedy available to them, being cancelling and refunding the reservation.
It could be argued that by altering the contract, Alitalia is in breach of contract. Had they voided the contracts, I believe they'd be fine, but they lost that opportunity once they altered reservations. For fun, I'm going to cite myself:
https://papers.ssrn.com/sol3/papers....act_id=2727403. My 2 cents.
Now, whether the ticket is subject to English common-law will depend on where the reservation was made (both physically and on what OTA). But I digress.