Change duty assignment after signing contract but no orders to station overseas, how difficult? Is this possible?

Has to be done through retention due to it being tied to your reenlistment. Branch cannot adjust retention requisitions. Definitely doable though.

First, if Fort Carson is the duty station of choice on your contract and you no longer desire that assignment, see your career counselor to waive your reenlistment commitment and deletion of assignment. A deletion of a retention assignment requires a 4187 endorsed through your command. Once your reenlistment assignment is deleted, contact your branch for the assignment you want now. Keep in mind there are rules that the branch must follow that differ from what retention can do. The reason retention will not entertain changing your assignment is because if every Soldier that changed their mind after they reenlisted about their location of choice, HRC retention would spend all their days trying to accommodate all the changes, it is not practical. The bottom line is see your career counselor.

I recall seeing Duty Swaps being advertised in the Army Times and always wondered how the hell that works...