Move Planning Customer(Account) from one Territory to other

Hey Experts,

There is a territory "X" that is assigned to Planning Customer. Now we want to assign Territory "Y" to same Planning Customer and deactivate Territory "X". How can we achieve this.

-Was able to add New Territory"Y" to Planning Customer but unable to deactivate/delete old territory assignment

-Tried creating new rule within territory "X" selecting validity end date for Planning Customer as current date

But still I am able to plan promotions using Planning Customer under territory "X" . I thought it would throw an error based on rule set that validity is ended.

Can someone please help how can we deactivate/mark territory inactive for a Planning Account. If there is any Note etc.

Appreciate any help.

Thank you