I'm sure it's not as easy as just saying "OK, I'll pass it on to William". When the reigning sovereign dies, the next in line becomes sovereign whether they like it or not.
There is no law that allows for abdication, when Edward VII did it an act of parliament was needed, a new one would be needed for Charles. When Edward abdicated he gave up any rights of succession for his heirs, if he were subsequently to have any.
The act directed that his brother Albert (who became George on succession) would succeed:
and accordingly the member of the Royal Family then next in succession to the Throne shall succeed thereto and to all the rights, privileges, and dignities thereunto belonging.
but the next para in the act
(2) His Majesty, His issue, if any, and the descendants of that issue, shall not after His Majesty’s abdication have any right, title or interest in or to the succession to the Throne
removes any right of succession to his heirs if he were to have any. He had no children, but Charles has, so the 2 paras would directly contradict if applied to this case.
All too complicated for me, I'm off for a lie down.