After some research and digging by Jayspec7, we've come to the conclusion that PDC can be retrofitted to non-PDC '04 3.0i and 4.4i X5s, as well as any non-PDC '05-'06 3.0i. Finally, this is great news for those wanting PDC for those model years!!! We will be working on a supplement for the retrofit guide posted by Kevin on the X5world home page.