Funny you should post, as I'm selling a WM5 update -
http://www.totalpda.co.uk/forum/show...6724#post16724
You should be able to update fine. A few applications will require a WM5 patch. Most sat-nav stuff will work fine so long as you have the latest patch.
I'm using WM5 and it seems to work very well. All the office appas have been updated, and the whole OS seems to run quicker than my old '02PPC.
Also there is a sticky regarding known issues, and fixes, which HP are addressing in the forthcoming ROM update -
http://www.totalpda.co.uk/forum/showthread.php?t=5480