I have a file that has a single task. its fixed duration and is not effort driven and no resource is defined. duration is set to 10 days. after 5 days, %complete is 10%. after entering this data and rescheduling, in MSP 2007 duration and %complete remain constant (10% and 10days), while in MSP 2010 %complete decreases to 7% and duration increases to 14 days! Why??! Another question: I set task type as fixed unit and other conditions remain constant as described before, after rescheduling in MSP 2010, task will split and finish date changes. (for example increases 4 days). Problem happens when I copy %complete column on itself. In this case, finish date returns to its original value (decreases 4 days.)
I am assuming you have done the following: Set the % complete to 10%, and set the status date to the end of 5 days. You have then used the Update Project - Reschedule uncompleted work command to split the task and reschedule the remaining duration.
In both circumstances, the finish date of the task is equal, so there really is no increase in the task duration. The fixed duration task in Project 2010 is "counting" the 4 days during the split - in Project 2007 they are not counted. If you look at the Tracking table, you'll see the Project 2010 task have 13 days remaining with 1 day complete 1/14 is 7% - where the Project 2007 file shows 9 days remaining and 1 complete 1/10 = 10%.
In Project 2010 with the default Fixed Unit task type - the task splits the duration stays at 10 days and % complete is still 10%. I do not know why you are getting a different value - other than to ask if you have SP-2 applied to Project 2010.
To the bigger question - Fixed Units, Fixed Duration, and Fixed Work really only have value when assigning resources. What were you hoping to accomplish using Fixed Duration tasks without resource assignments?