OK, sorry.....
If you want existing FP 'webbots' to work properly and fully (includes photo
gallery, navigation bars, site maps, site search, include pages, shared
borders etc etc), then you need the Server Ext installed. Expression quite
happily works with existing elements such as those; but you can't add any of
those particular features within Expression.
You could keep Frontpage as a 'backup' incase anything you needed, and
insert it into your pages from Frontpage - you can interchange between FP
and Expression without any harm to the pages since they both use the same
methods such as opening a site (File > Open > Site and publishing in
Expression is the same or similar to that in Frontpage including HTTP
publishing.
If you don't need the server ext you will then have to publish by FTP mode
frm Expression or with Cute_FTP or whatever prog you use.
I would keep the server ext in place for the time being until you learn
Expression fully. There are lots of neat features that ASPX .Net 2.0 offers
(but requires windows server). You can still develop the site in
Expression, taking advantage of its CSS support, and then use the equivalent
scripts for your site features (forums, guestbook, gallery etc) for a
Linux/Unix server (like PHP or Perl/CGI).