The journalists/media who are saying the squad numbers can't be changed after the start of the season are correct. Cavani can't just surrender his no. 7, nor can Mejia surrender no. 77, nor can Pellistri surrender no. 28, unless numbers can be taken off players who are on loan - that's been done in the past, but the EPL rules make no mention of it currently.
So unless the EPL will give a dispensation for purely financial or sentimental reasons, CR will have one of the unused numbers such as 24 or 32. There aren't that many to pick from, as we're currently using 69 squad numbers.