When I said that I think there should be a zoom-in / zoom-out slider, what I had in mind was that the caption would update accordingly.
For example, at the default position the caption would say "bitcoin", but zoomed in a few decimal places would say "millicoins" (or whatever name we agree upon).
I tend to agree with this. While any change is bound to cause some confusion I think the idea of an option to set how you view your bitcoins would be much easier than a sudden all out sudden change. It seems to me it would make for a smoother transition.