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prophesying is when you tell people that God said something to you...
if God changes what he said he would do...then whatever you prophesied has been revoked...
For example. God said he would destroy ninevah if they did not repent. They did repent, so he relented in destroying ninevah. That was not God revoking prophesy, that was God revoking what he said he was going to do.
This is much different then saying messiah will come at this time, in the place. at this time. Etc etc..
a good example is hezekiah's illness and isaiah's prophecies about it...