Which fallacy is that, exactly?
And what does this above quoted post have to do with your changing past tense to future tense?
It seems that it is self explanatory.
Look how the same prophet worded another prophecy:
“He was despised and was avoided by men, a man meant for pains and for having acquaintance with sickness. And there was as if the concealing of one’s face from us. He was despised, and we held him as of no account.”- (Isaiah 53:3)
Now this was written long before Jesus came to earth and the prophecy was fulfilled. Yet the same prophet referred to this event as already having happened, past tense. Why do you think that is?
Bonus, here is another prophecy that had yet to be fulfilled written in the past tense, by Enoch, even prior to the flood:
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Yes, the seventh one in line from Adam, Eʹnoch, also prophesied about them when he said: “Look! Jehovah came with his holy myriads to execute judgment against all, and to convict all the ungodly concerning all their ungodly deeds that they did in an ungodly way, and concerning all the shocking things that ungodly sinners spoke against him.”-Jude 14, 15.
Why do you think the prophet wrote this past tense as if it had already taken place?