Caesar was a man
Caesar is not (any longer) a man
Some man is not a man
It's not, unless our domain of quantification consists only of men. But I can easily restate the argument
No y is identical with Caesar
Some x was identical with Caesar
Some x is not identical with any y
I don't see any way round that. Of course, some x was identical with some y (for y = Caesar, where the verb "=" has past tense). And perhaps we could read the existential quantifer as tensed - there was an x such that x is not identical with any y. But there's no way we could make any sense of it in standard predicate logic. Moreover, the standard way of understanding quantification as a kind of relation between variables or open sentences or predicates on the one side, and objects on the other, makes no sense either. For example, logicians say that the predicate "- was an emperor" is satisfied. Is satisfied? Or was satisfied?