The solution to many vexing grammatical problems is to simplify the sentence, sort of like reducing an algebraic equation before solving for x. So that phrase becomes “Not one is upgradable.” (Oh, and by the way, both “upgradable” and “upgradeable” are valid.) Well, that scans. One (singular) is.
Except, wait, do I really mean none as a singular? When I’m really talking about the cluster of options for the MacBook Air, each one of which is individually upgradeable?
Some days it’s fun working with words.