I registered for the SAT Subject test in Physics, but I think I'm going to take Chemistry instead. Can I switch from Physics to Chemistry? What are the cons?
Yes, you can switch from Physics to Chemistry on the day of the test. All SAT II test booklets include material for all tests (with the exception of language tests with listening), so you can just take the ones you want.
The only con would be if you started to take the Physics test and realized you didn't want it scored after all, but are still taking other tests that day that you do want scored. As the College Board states on its website, once you start to take a test, that's it - it'll be scored...unless you cancel all the tests you took that day. Of course, you don't have to report the score of a test you do poorly on to colleges (unless they require all scores be sent), so this isn't really a huge issue - you can just take the test again on another date, or register for another subject test on another date. There's also the slight possibility that your testing center would for some reason not let you switch from Physics to Chemistry (since College Board reserves the right to deny the switch for its own reasons), but it's highly unlikely that that would happen for a switch between two non-language-with-listening tests.