Hi there--
If I understand correctly, You want to "assign an other resource to the assignment using PSI"
Please refer to article of PSI limitations:
http://msdn.microsoft.com/en-us/library/ee767706.aspx
What the PSI Does Not Do
*Changing the resource on an assignment. However, you can use the PSI to delete the assignment and create a new one.
Thanks, Amit Khare |EPM Consultant| Blog: http://amitkhare82.blogspot.com http://www.linkedin.com/in/amitkhare82