It's true that the TA/prof can lower your grade as well as raise it, upon remarking.
I think it's standard "protocol" to talk the TA first. Chances are, they won't mind remarking it. But if they feel it has been marked fairly, then you just visit your professor during office hours or send an email to set up a time to meet.
I've only had one essay remarked at Mac - and it went from a C- to a B+! So it's definitely possible to get your mark changed. Just keep in mind that it can be lowered too.