How can Starter pets be regens????

So long as a pet falls within the parameters described here (Bonus Section 1 Regennable Pets, parts 1 and 3), and is one of the species that has been revamped since the regens became a Thing, a starter pet can be a regen.

As an example, our own starter Munir became regennable because spark stripes was split into two trait slots (G1, body, and G2, extremities) when it was previously one trait (we chose to regen him with the body spark stripes because we were always pleased with his design ad wanted to keep him as similar to his original design as possible.)