When a post is migrated from one site to another, the revision history is lost. I think that it's important to retain this history even if the post is moved to a different site.
There is also a practical reason for doing so - sometimes a post is edited to say "please move me to X" in order to get other users to close the post as "belongs on X" or to get a moderator to move the post themselves. While this is an acceptable practice, the post comes out on X with the phrase "please move me to X" at the top. Since it's no longer necessary, removal of this phrase is warranted.
This is, however, where the lack of a complete revision history shows its weakness. Although the post displays the latest revision, when the post is edited the content in the edit box is from the first revision, which does NOT contain the phrase we're trying to remove. As long as the only thing to change was the removal of "please move me to X", it's safe to add some dummy text (as the edit doesn't go through without some change) and then roll it back.
If the latest revision differs substantially with the initial revision (as I expect the FAQ questions will) then you need to go to the site where the post was migrated from, get the source of the last revision, and make your changes on that. (this is, incidentally, what I did on this post). It's a highly manual process, and one that can and should (for the sake of consistency) be avoided, and it can with the inclusion of the entire revision history.