How does one change to engineering for graduate school after undergrad in a science field?
I will answer a slightly more general question.  How does one change fields between undergrad and grad school?  The answer very carefully is correct, albeit unhelpful.  The single most important criteria when applying for a strong PhD (at least in math, engineering, or the natural sciences, but probably true in other fields too) are letters of recommendation and demonstrated research ability.  Bad grades can sink your application, but good grades (even from a top school) are not enough to get you in (nor good test scores, although they help).  

If you want to get a PhD, but don't have any experience in the area, you should first apply to a masters program, where you can gain experience and impress potential letter of recommendation writers.  Often admission to a masters program is less competitive than to a PhD program (if even getting into the masters program is difficult, you could start as a non-degree student).  But if you can publish a few papers during your masters and convince a few of your professors that you have both the ability and desire to succeed in grad school, then you're application to a PhD program will be much stronger.  I also recommend that you talk with the schools where you plan to apply and ask what they suggest that you do to strengthen your application.

You might also like to read this question and answer Is it possible to attend graduate school in pure math after undergrad in CSE?