a first level Tendersoul knows all the spells on they're list.
However, she must choose six orizons, and two first level spells. She has these spells as spontaneous casting from now on.
Say, three days later. She decides she needs a different spell. At midday she goes to a sunny clearing/backyard/temple to the Goddess and prays, she can now reselect her eight spells, which replace her old selections as spontaneous casting.

The Pureheart still can't die, mind you. They just return with fast healing 1 instead of 10 until they recover to 1+ hp, in which case they're normal 10 fast healing kicks in.
but sure, changed hit die for the Pureheart to d8