Nothing: this has always been an easy concept go grasp. It was the easiest thing to understand.
Zero was much longer in coming.
Nought was an intellectual breakthrough.
Reading this over, I’m reminded of the Abbot & Costello sketch ‘Who’s On First?’
Zero: is what it should cost you to update your will.

Zero: The Cost of Updating Your Will

For centuries, the lack of the number zero was a bar to sophisticated mathematical calculations. Today, the concept and symbol of zero are widely understood. We can perform calculus and perform complicated calculation and have computers.
Zero. Many books and doctoral thesis are being written on the subject. But so much for the purists and the academics. Zero is a useful concept in keeping one’s will up to date – it is a central part of the service my clients get.

Out of date wills and testamentary documents are useless. Sometimes they are worse than useless.
Often, they are worse than useless. Out of date wills have been the foundation of some of the bitterest of inheritance disputes.

Zero helps keep all my clients wills up to date. When I update a will, I charge the grans sum of zero. That way, my clients’ wills are always up to date.

In sum, all things being equal, the cost of updating your will should be zero.