People do not upgrade/migrate from WSS to SPS.
SharePoint Portal server is a product that sites on top of WSS. It is
designed to index, categorize (organize), and present information that
is typically stored somewhere else (like WSS sites). Some content, like
news and other typically static, enterprise level data, can be stored in
the portal but that is typically the exception rather than the rule.
If you are just hoping to move your Site Collections from one server to
another, you could accomplish this with many tools. FrontPage FWPs may
be the easiest way to choose what to move where.