If someone has already voted on a question or answer, they can't vote again just because it's been edited. They can change their vote (i.e. remove an up or down vote) when it is edited, but they can't add another upvote if they already gave it one.
Editing bumps a post back up to the top of the active list if you are not within a short window of the last edit you made (5 minutes). For example, if you post a question, then edit it within 2 minutes, it won't get bumped back to the top.
Sometimes only small edits are needed, like spelling or capitalization. If you see someone do this once or twice, that's not unusual. If you see someone continuously making minor and unnecessary changes in an apparent effort to keep their post at or near the top of the active list, this is generally considered abuse. In such a case, I generally leave a comment or flag it for moderator attention.
There is also an auto-bump that is routinely done by the Community user to give old unanswered questions more attention.
If the question itself is migrated, and the same people who voted have associated accounts at the destination site, then they can vote again. But the votes before the migration will no longer shower with reputation and instead go to the Community user.