They are artificial creations. Like our current tech, they are designed using modular components, and they accept updates in the field.  My Blu-Ray player or dishwasher downloads new firmware; Industrial enginines by [IH](https://en.wikipedia.org/wiki/International_Harvester) can have features like total horsepower *unlocked* or configured by software [while in-place](https://en.wikipedia.org/wiki/JTAG).

So consider the “upgrade” to be designed as something like an in-app purchase from the original maker.

With the loss of original maker and the (actual) evolution of the forms as they became feral, the code is a bit mixed up, like with the [*Dragon’s Teeth* of Mirabile](https://www.goodreads.com/book/show/1141442.Mirabile) by master world builder Janet Kagen. Rather than buy an unlock code, the needed triggers are found by side-effects or evolved access to useful survival traits by the feral form.  You might read Kagen's stories to bet a feel for what I have in mind about the DNA.

Being modular, distinct traits like limbs can be recessive or switched off or damaged in a feral remixed offspring. Many features are simple configuration options using common code.  Although inspired by legacy bioforms that were unrelated and incompatible, the engineered lifeforms used inter-compatible standardized traits and developmental configuration programming.

Now in ourselves such a change like extra toes would only be effective in the germ line, as development occurs once. But these animals were designed for having upgrades applied. Re-development — metamorphosis — was **designed in** so changes could be *applied* to existing phenotypes.