Once again I thank you very much. Technically the 850_2820 doesn't work with the v7.450 software.
However, I stumbled on a thread after I had posted that confirmed v7.910 (9185) works on WM2003. I was not aware of this.
I have updated both the core software and the map successfully, so its now WM2003, v7.910 with the 850_2820 map.
I have left this as a reference to others in need of the same info.
Just to add, I think the highest software version that works on WM2003 is indeed 7.910, and the current highest map version is 850_2820.
(If the map version increases, please state on this thread and I will edit it)