<p>Some colleges like the UC's require 2 SAT subject tests in 2 different areas of study.
I believe you can change your test...by calling college board or something or maybe change at the location for a fee. like 20 some dollars.</p>
<p>You can change it on the test day; don't worry. This is unless you're switching between tests that involve listening (language tests).
You can also add/drop subject tests on the test day.</p>
<p>not a requirement to have 2 sciences...I mean if you think you're strong and can do well in 2 sciences go for it, but it's definitely not required. Also, having ONLY 2 sciences and a math may be a little lopsided..that is if you werent planning on taking anymore. Try to get a history or lit in there too if you can.</p>
<p>You don't have to get confirmation or notify CB of any changes, you can take any other test unless, like k twin said, it's one of the listening/language tests.</p>