Now why did I want to do this? The Dell Axim does indeed run well under PC 2003 as people say, but I ran into some things that made me want to switch. I received a Bluetooth keyboard I wanted to use but there was no support for it under this OS according to the makers of the keyboard. I could not get Google applications for Gmail and such to install and run. I suspected that connecting to free WiFi at coffee shops might be easier. Some programs I wanted to use did not work under PC 2003. I don't remember clearly, but perhaps the Windows sync software under Windows 7 wouldn't sync properly, but that may not be true, anyway I finally managed it.
I started with the advice on this page and got nowhere upgrading from Windows Mobile PC 2003.
By all accounts you need to go from 2003 to WM5 before using any of these WM6 updates. The upgrade listed there to go from 2003 to WM5 ROM A01 would not load and the full program for going to A02 would not work at all. Fortunately I found a like program for going from 2003 to WM5 ROM A01 that did work. After that, things worked as advertised. Here are the steps I went through:
1) Installed ActiveSync 4.5 on Windows XP. The Mobile Sync software for Windows 7 was not working for me at all because it kept throwing up some error about ROM updaters not working etc.
2) Ran the software I found for updating Dell Axim x50v to WM5 A01. There is a clear message about canceling a notice from ActiveSync that I ignored the first time through so I had to do it again.
3) Ran the software found on the XDA Developer's site for upgrading to WM5 A02
4) After that, the upgrade images would load from my SD card as advertised.