You'd have to be doing pretty poorly to be forced to move down.
Seriously, I don't understand why you'd desperately want to do a subject if you're shit at it. It's much better to ace general maths than fail 2 unit. But if they're just making room for 3 unit people, don't change. Go to the class and they can suck on it. It's also perfectly possible to complete 2 unit in a 3 unit classroom, you just don't do some of the work that they do.
And nobody wants to do more than 12 units. It's a lot of units and it basically means you're doing 3 for no reason, because they won't count.