dnd 5e – Can polymorphic monsters spam their abilities?

Some creatures (like Couatl's "change shape") have a capacity that makes them polymorphic. I wonder if this ability can be used over and over again when the form is dropped to give the creature a massive effective health spool.

For example, if the Couatl used their form to change shape and become a veteran, then enough damage was done to kill the polymorphic form. Could she then use the reshaping ability again to become a fully healed veteran again?