I really don't know if the ROM will work on the older XDA II's. I would hope for the rest of you that they do. The difference between the XDA II and the XDA III,IIi and IIs is not enough to switch when the price tag is most likely 1000$ US.
As for the improvements not being worth the upgrade in the US, that statement does not hold true at all. The XDA II's and III's are fairly worthless in the US since they do not support the 850mhz signal. AT&T and Cingular are lying bastards when they say they are still developing the 1900Mhz signal. They are not, and they are in fact converting most of the 1900Mhz towers to 850Mhz.
Bottom line is that your only hope of getting a working cellphone in the US now is on the 850Mhz GSM band or CDMA through Verizon. TMobile sucks #*#$& along with Sprint PCS which really only leaves Cingular/AT&T which will become one provider soon anyways.
I know there are some TMobile users out there that will rush to defend, but I am only talking from my own experience where I live with their 1900Mhz GSM network.
The XDA IIs and IIi are the answers to my prayers. Not only are they XDA and totally kick #$#, but they will actually work with my provider.
Some people swear the XDA III's will actually be Quad-band and not Tri-band. All I have heard are rumors though, not detailed specs. Everybody who has tried to correct me has published the links to the XDA IIi and IIs, but those are only BASED on the XDA III model. The TMobile MDA III specs and QTEK 9090 specs (Blue Angel) indicate Tri-Band still.