Well, you can find the topic list for Calc BC

here. The topics with a + or * next to them (starting on page 12) are the topics in BC that aren't covered in AB. As you can tell, the main differences are that 1) BC covers derivatives and integrals in polar and parametric coordinates, 2) BC covers slightly more applications of integrals to various sorts of problems, and 3) BC has a long section on series expansions.

Topic 3) is going to take some effort to learn no matter what, and you'll be best prepared to tackle it after understanding all the stuff about derivatives and integrals that came before it in class. However, 1) and 2) CAN be made easier because they're basically just trying to get you to combine stuff you learned in pre-calc with the stuff you will learn in calc. The better you know the stuff from pre-calc, the easier your life will be. For example, do you remember parametric and polar coordinates? Trig identities? Basic geometric facts like the Pythagorean Theorem? If so, and if you can at least keep up with the calc stuff you're learning, then the additional BC topics will probably be no harder than the AB stuff.

Keep in mind, however, that calculus really is a very layered subject. The better you learn the stuff about limits at the beginning, the easier it'll be to understand derivatives. The better you learn stuff about derivatives, the easier it'll be to face some of the harder problems with integrals, etc. The stuff about series expansions (Topic 3) comes at the very end of all this, so if you want to do well on that part, make sure you've worked hard the rest of the year.

But despite all this, I really think you don't have much to lose by taking Calc BC (the class), unless you're one of those people who's deathly afraid of math. If you're marginally interested in a career involving engineering, math, or science, the stuff in BC is only the very beginning of what you'll be learning in college. If you do well in the class and ace the exam, well, you'll have a head start in college. If you don't manage to get credit, then at least when you take the class again you won't be seeing it for the first time, and you'll be able to solidify the stuff you learned before. Win-win either way to me. You might be worried that BC will hurt your high school grades compared to taking AB, but honestly, do you think that 3 additional topics in one class is going to make the difference between getting amazing grades and getting into all your colleges, or having a terrible term and getting nowhere in life? It might be possible, but you probably wouldn't want to put any money on it. There are other factors of course, like how good is the teacher, do you work with other people, are you motivated, are you taking lots of other AP classes, etc., and you'll have to take all that into account. But assuming all those other things are pretty reasonable, I doubt that switching up from AB is going to be the end of you.