I just shifted from ECE to theoretical physics. Apart from taking the standardized tests it will be better if you could do atleast one research project in physics. When I applied to universities, I already had 3 summer projects in my profile, and I guess that's the main reason why I got admit from most of the universities I applied to. If your summer project turns out well, you could also get a good reco from your project guide. Further, if you could get a publication from the project, your admission in some good university is almost assured (assuming you also have good GRE scores).