This one is pretty easy to get but the language confused me at first. You are not replacing the actual location of the mystic, you are replacing a stat that is currently on an item you have with something else.
To get this achievement complete the game until you reach Act V and acquire the Mystic. Once the Mystic is back at the camp talk to her and select Enchant. Choose the item that you want to change and select the stat. The menu will ask you to confirm that you want to alter this stat and tell you that once you alter this stat you will be unable to change any other stat on this item. Confirm the change and you will be given a list of possible changes to make. Select the one that best suites your build and confirm once again. Now the stat has been changed and the Challenge/Achievement should POP!!.
Hope that helps,
Let me know if there is anything that should be changed about this guide.
Stay Awhile and Listen.....