This explanation refers to the following four edges:

Cooshee Form
Pseudodragon Form
Hook Horror Form
Cave Fisher Form

In addition to their other restrictions, these edges are mutually exclusive with each other.

The form swap edges are similar to Spin the Wheel and Roll the Bones in that they allow a shifter to exchange a particular kind of new and as-yet-unused form for a different form. However, rather than provide a different random form of the same focus and tier as those edges do, these edges instead provide a specific alternate form.

Cooshee Form allows an elf shifter to swap a 2nd tier offense or utility form for the cooshee form.

Pseudodragon Form allows an elf shifter to swap a 2nd tier air form for the pseudodragon form.

Hook Horror Form allows a svirfnebli or dark-elf shifter to swap a 2nd tier offense form for the hook horror form.

Cave Fisher Form allows a dark-elf shifter to swap a 1st tier utility form for the cave fisher form.

Although these forms (like most forms) have some cool combinations of abilities that make them unique, overall they are balanced with the forms they replace. You shouldn't be looking at cave fisher form and imaginging a dark-elf shifter stomping his whole PK range repeatedly with it (probably) -- it's not a quest form (although, like a quest form, shifters can't draw it randomly), it's about as tough as the other 1st tier utility forms.

I won't rule out the possibility that we will introduce some kind of form swap edges for shifters other than of these three races in the future, but at this time the idea is primarily to make elf and dark-elf shifters at least interesting. There isn't much of a mechanical argument for playing either at the moment; we'll see how much the prospect of getting forms you can't get in other ways changes that, if at all.