I thought title holds quite some weight in this respect, but maybe not quite that much... ?
Anyway I think Misi was refering to renaming the file itself... which I did try :) once. Result: PR lost...
Well, yes, I do suppose you would lose your PR if you change the page's filename. I don't think it would matter if all the content is the same, the backlinks and such would still be pointing to the old file/page.
Renaming a file is the same as deleting it as far as PR is concerned. As correctly pointed out above, all your old inbound links will still be pointing to the old page.
Modify your page, modify the title by all means - but why would you want to change the filename? It's a bit like moving house and wondering why you're no longer getting all that mail you used to get from your subscriptions! If you don't change your address with them all, they will have no idea where you moved to, will not be able to find you, and the SE's simply decide that the page does not exist anymore.