I didn't mean to dissuade you from doing the swap-over, just warning of the amount of modifications you will have to make - especially when it comes to the product and check-out pages.
If it were me (but it's not) - I would accept the basic structure of the PowerStore template, and change just the colours, images and background images, and then work on the individual page elements one-by-one, testing at each stage.
If I wanted to completely change the structure to a narrower template , I think I would make sure that I could modify the existing product results, detail and checkout pages (and their repeat regions) and category menu in the original first (checking at each stage) and then copying those modifications over to the other template bit by bit.
But that's only because I would worry that my modifications might interfere with the dynamic functionality of the pages, and I don't know enough php to easily put things right if I mess up.
So I suppose I was only projecting my own worries about that - I haven't tried a straight template-swap so haven't thought that far ahead. It might not be quite as problematic as I imagine. Maybe others have tried it and may want to chip in.