At the beginning of each upkeep, if no spells were cast last turn, transform Ulvenwald Mystics.
For more information on double-faced cards, see the Shadows over Innistrad mechanics article (http://magic.wizards.com/en/articles/archive/feature/shadows-over-innistrad-mechanics).
You can regenerate Ulvenwald Primordials in response to the triggered ability that would transform it. If you do, the regeneration shield will apply to Ulvenwald Mystics that turn.