<p>If you go to grad school for theoretical physics, they will expect you to have completed the coursework of an undergrad physics major. Basically, you would have a double major, i.e. physics and engineering. That seems like quite an endeavor.</p>
<p>alchemist said it pretty well. The only place I might disagree with alchemist is that schools may want to get you right into the PhD program (I looked at Princeton and MIT) after undergrad.</p>