If you keep the same file name (index.html) and I like to keep the head part (title and meta) in tack it should not affect your current PR but long term it all depends on what you ad or remove from the page as far as content goes
I think if your filenames kept changing and the content in them constantly changed then it probably would hurt it. If the Major content of the pages remains constant and you're only changing look and feel it should not hurt. If the basic content is in flux then the spiders would never be able to figure out what your site was about. For that matter, your linkers would not either. PR is only useful if it leads to visitors. If people are jumbling around their content and making it confusing for visitors too then folks will just go elsewhere.
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