Why Can't I Change My Coupon/Promotion to be 'Per Customer'?

Is it possible to change a Coupon/Promotion from 'Unlimited' to 'Per Customer'?

No. After creation, Coupons and Promotions cannot be switched from 'Unlimited' to 'Per Customer'. 

Why?

It causes a conflict with the existing Orders using the current Promotion/Coupon. 

What's the Solution?

The best solution is to Duplicate the current Promotion/Coupon and configure it to be a 'Per Customer' limit.