I have an established web site for about 5 years which is PR4. I'm wondering what would happen if I completely changed to context of the web page, with regards to indexing already done by Google, Yahoo, etc.?
You will keep getting traffic from 1-2 months and then it will start decrease because google index pages time to time. So, Your PR will also reduce after next PR Update.
I do not think that it will effect your PR . Because it mostly depends on the your site popularity and the kind of visitors, so if your keep the back link as it is then it will not much effect.There might be a slight difference in position.
if might effect your PR, because if you have back links to your deep page and if the page gone, a 404 error will let search engine bot not counting the passing PR juice