geo12,
You can't modify the history with the current version of the program, UGNX2, even with the best method of exchange -the .xpk file. What UGS intends that if you need to modify the part, you go back to I-DEAS to do it. Then you re-export the new .xpk file and update the part in UGNX. It is a fully associative change, i.e. if you have toolpaths, they update. The reverse is also true - modify the original part in UGNX and update in IDEAS with associativity.
I was at the recent PLMWorld conference and they demonstrated UGNX3 - which is not released to the public yet. It is the first version that, if you really wanted to, you COULD completely migrate to UGNX3 from I-DEAS The parts are fully modifiable, same with assemblies. It is more complicated than just exporting a part from IDEAS and importing to UGNX with history. There are a series of steps, but it can be done. You'll need to contact your UGS-PLM rep or your VAR to get all the details.