Manually Change CiviCRM ID?
If there is a contact (y) with that ID already then one option via the UI would be


create a new contact (z). Then merge z and y so that z inherits all the information for contact y
retrieve y from trash, and go through the same steps of merging y with x (your original contact) such that y inherits all the info from x.


Outcome should be that your original contact will now have the ID of y.