How difficult is it to change your area of scientific research?

I've managed to putter about in a couple of zoological fields and even a few non-zoological ones. You sound as if you are interested in several disparate fields and can't decide among them. You can chase the grants if you want but the chance is that, by the time your field of research "goes out of fashion", you will have tenure and can concentrate on teaching or doing research that doesn't require grants. Your university will be stuck with you unless they can prove "moral turpitude", and I've heard that, for that, you have to be caught committing under a street-light with three witnesses. In other words, it's quite rare; I know of only one instance and that was a bit bogus.