What if people vote down my question(s)/answer(s) and then I lose some rep points (below the level required to vote up), and the privilege of voting up goes down the toilet with it (until I regain the reputation points needed). Do the up-votes I already gave other Q&As disappear, lowering back that user's reputation in the process (I mean the author of the question/answer I voted up)? I think the answer is a bit self-evident, but I can't be sure.
Do the up-votes I already gave other Q&As disappear, lowering back that user's reputation in the process […]?
No. The votes you once cast can not be removed like that. The only exception, where votes "magically" disappear, is when you use your votes in a non-constructive way, e.g. by only voting for one user, or aggressively downvoting specific users.
When you lose a privilege, nothing retroactive is done. You just don't have the privilege anymore, simple as that. Similarly, when you lose the editing privilege, all your edits stay.