Actually, I tried the custom install and selected only new content, since I was planning to leave my P6 runtime intact. I then found that nothing in the Walk Designer got installed, and I couldn't open the Walk Designer window, even with the P6 runtime attached, because it couldn't find a file. I went back and did a full install, and that problem cleared up.

I've been planning for a while now to move my runtime to another disk anyway, so I'm think I may just go ahead and re-install my content into P7's runtime anyway.