For the subject tests, the old is SAT II, and the new is SAT Subject Tests.

Before 1995 there was the SAT– what that actually stood for changed a couple of times (achievement to aptitude to assessment, and now I think it doesn’t technically stand for anything, lol). There were also the SAT Achievement Tests prior to 1995.

After 1995, the test was “re-centered”– students were doing collectively worse on the test, so they decided to change the way they turned raw scores into scaled scores to make the average right around 500 on each section again, like it was supposed to be. I don’t think they actually changed the content or difficulty of the test. However, they changed its name to the “SAT I” (they might have called it the “SAT I: Reasoning Test” for a while, from what I can tell), and made the SAT Achievement Tests called the “SAT IIs” (or “SAT II: Subject Tests”). The SAT I consisted of a “verbal” section and a “math” section, and there was a separate SAT II for Writing which was entirely optional (like all SAT IIs).

In 2005, the SAT I was significantly changed, largely because of criticisms about certain parts of it– mostly (I think) the analogies, which were removed entirely from the New SAT (I personally liked the analogies and was sad to see them go). This new version is called the SAT Reasoning Test (or people just call it the “New SAT” and call the pre-March 2005 test the “Old SAT”) and they renamed the SAT IIs “SAT Subject Tests” (although people still often call them “SAT IIs”). The new SAT has three sections, the critical reading, mathematics (the technical change from just “math” to “mathematics” amuses me for some reason), and writing sections, and features short reading passages, a slightly different range of math coverage, no analogies, and (I think) more fill-in-the-blank vocabulary questions (I actually don’t remember if the old verbal section had fill-in-the-blank; in any case, they were determined to keep the vocabulary testing even though the analogies were the most criticized part of the thing), as well as (I’m sure) some other changes. According to Wikipedia, they made it a bit harder on the whole to correct for the increasing number of perfect scores.

To clarify another thing, the SAT Reasoning test is essential to college admission (well, or ACT counts for about the same thing, if you like) while the SAT Subject Tests are only required by some colleges and universities for admission, and you can choose which subjects you want to take (you can either try to look well-rounded, or show strength in areas related to your intended major).

That might’ve been way more information that you wanted or needed, lol.

By the way, there are also SAT Subject Tests. You can take a Subject Test in one of the following subjects: English, biology, chemistry, physics, math 1, math 2, US History, World History, and various foreign languages. These used to be called SAT 2s, but at the same time as the name for the SAT 1s was switched to the SAT Reasoning Test, the name of the SAT 2s became the SAT Subject Tests.