You don't really register for a specific subject when you register. It looks like it, but it's not really so.
When you go to take the test, they give you a booklet with ALL of the SAT II exams in it. You are allowed to pick and choose which ones you want to do on the day of, so if you want to switch from what you said at registration, you can.
Options include:
1) Switching the subject of the test you picked at registration (Like physics to US History)
2) Taking less tests than you registered for if you feel worn out after one (You just don't get your money back)
3) Taking more tests than you registered for if you feel up to it (College Board will just send you a bill for the extra tests)