As far as I know, Google PR of your front page (or of any page) is mainly affected by the number of NOFOLLOW links to your front page Google can find in the WWW.
So I don't see how could you loose so much PR by simply replacing page contents.
What you might loose is Google search results positions that obviously depends much also on the page contents, I mean a page talking only about "cats" is difficult that shows up when someone search for "handpainetd glasses" even if the page has a huge PR.
So can't you just ftp download your current front page, and back it up on your PC, than you substitute it with the new page. If after some months you see that you are loosing positions on Google search results for the search queries you are interested in, you just simply ftp upload back the old front page, that's it, it will take some months, but there is not much you could do.