Adult Blue Dracolich have a CR of 17 (should be prof+6), but all of its modifiers count as it have a prof of +5. The dracolich and demilich issues are both in the queue for correction in a future printing of the Monster Manual.
— Jeremy Crawford (@JeremyECrawford) June 28, 2017
So, for the moment… Dracolich becomes CR 16? You can use the dracolich as written. Any correction we make won't change its CR. It's functional.
— Jeremy Crawford (@JeremyECrawford) June 29, 2017