On page 113 of Princes of the Apocalypse the text reads in part:
As the tremor ends, a young purple worm with 184 hit points (challenge rating 13) burrows…
No other changes to the Purple Worm’s stats are given and the monster does not appear in the Monsters section at the end of the book. This monster also shows up on DnD Beyond’s encounter builder as a CR 13 monster as "Young Purple Worm", in contrast to the CR 15 "Purple Worm".
Is taking away 63 hit points with no other changes to this monster enough to drop it two challenge ratings? I’d like to challenge, but not kill, my party.