Hmmm. 
I don’t think there’s a way to direct a post to a different/specific reply after it’s been published. There maybe some scope to add a new post directed at the right reply with a copy/paste of the original text, and then ‘change ownership’ to the right user? Or possibly some ‘move post’ juggling? None are particularly elegant solutions though, and I think a lot depends on the current format/sequence of the FAQ topic.
Have you got a link you can share?