<p>That is true to an extent. You have to take first year engineering before you can go into a field of engineer, so yes you need to start as a freshman. If you went to another school and took one year of engineering and then transfered to Purdue, you could go straight into one of the disciplines.
Once you take first year, then lets say you go into mechanical, then after sophomore year, you want to switch to aeronautical: I heard that was easy to do, but obviously takes more time.</p>
<p>As for the transfer from something else to engineering, I have no idea.</p>
<p>With the ACT, that is what they are looking for: high math and science. Those APs look good, too.</p>