I suspect the HP Jornadas can not be upgraded since the new chips, that work with the Pocket PC versions of the OS, differ in type and run faster.
I am actually reluctant to suggest this, since I know how annoying it is to buy new 'expensive' hardware when you have a quite capable machine already, but you would most probably be best off upgrading your hardware in the long run. If you believe your current PDA has been trusty, served you well and is worth its weight in Gold, then perhaps it is time to give it a rest. :-) Pocket PCs are extremely good and the HP H1900 series is incredibly good value for money at present!
HP official releases, for example, state that they are allowing Pocket PC 2002 versions of the OS on HP PDAs, to be upgraded to the Pocket PC 2003 OS only on the higher end models of the current series range! Therefore this is why I presume the Jonardas will not be upgradeable. I may be wrong in that an upgrade could be available to turn WinCE into, at least, a PPC 2000 for the HP Jonarda, but I doubt it.
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