Friend of mine has a 955C serial # 12A-1844 and is wondering if these units still used the D4 based undercarriage, as he's found a 7U D4 with a bad engine but would pick it up for the tracks if they'd fit - aside from the 7U chains likely being shorter than the 955 used but parts is still parts.  From the reading I've done to this point, I believe the change to D6 sized U/C didn't happen until the 955E model at #12A-5793, but wanted to get confirmation on this from people who would actually have experience with these machines, as I don't have any of my own in this department.  Thanks in advance! 

Change at 12A-5793 is correct.

Thank you for the help OM, I'll pass the info along