If a CR10 Deva uses Change Shape at half health to assume the form of a Stone Golem, do they become a full health Stone Golem? Is there anything preventing this same Deva from changing back into it's natural form when the Stone Golem is about to die, and making its last stand with that (I presume) half-health it initially had in its main form?
Does Change Shape give such monsters an effective HP buffer, or second health bar?
This question presumes the Deva knows the full extent of its powers, and intends to use them to their fullest. (While still fitting the CR10 category for threat!)