Don't know of any way to set constraints without the part moving in compliance with those constraints.
Also no way to keep offset values for a constraint when you change one of the references - it always makes the updated constraint "coincident".
It doesn't always do this for me, so it must be some sort of function of distance between the parts when the part is added and the overall size of the assembly.
If you go into the Placement box and uncheck the constraint enabled box, the part will not move as you are setting the constraints.
The problem with that is I'll need to go back and check that box again. Old Pro/E had a checkbox called Preview that could be unchecked.
A couple of different solutions come to mind.
If none of these are solutions, could you explain how you would like Creo to behave? I.e. describe more in detail why the behavior is an issue.