the upgrade path with pocket pc is that of the macintosh. your pda may support a new version of pocket pc, but then it may not. that is, upgrades to a new version is solely dependent on the decisions of hte manufacturer of your PDA. and unlike upgrades before, you are reinstalling the operating system via ROM. and you LOSE all your data no matter what you do. (backup).
you buy pocket pc 2002 or pocket pc 2003 from the manufacturer of your pda. you dont buy a "generic" version due to the massive amounts of customization required for each pda to work.