When Sliders began season four, it was quite apparent that the second season Kromaggs were nowhere to be seen. Instead, some new breed was on the scene...yet, they were claiming to be the Kromaggs of old? How can this logically be explained? Well, here are a couple of theories...
The theory which has the most merit is based on the concept of nationality.
The image above comes from the brief look at Kromagg history given to us in the "Genesis" Online Slide. It is explained that the Kromaggs dominated the Eastern Hemisphere of Pacifica and their history played out much like ours did...ultimately culminating in a Kromagg sailing the Atlantic Ocean to the New World. The Kromaggs believed this new land to be the fabled lost continent of Atlantis, and its lands were populated by humans (much like the New World held Native Amercians on our Earth).
With this in mind, we have to remember that the Eastern Hemisphere is made up of several races of people who all have their own distinctive look (which is noted in facial structure, eyes, hair, etc.). This is what I believe the season four change can be attributed to. In second season's "Invasion," the Kromaggs had a look very similar to that of ancient Orientals. They had long ponytails reminiscent of the Samurai, uniforms which resembled that of the Ninja and even their facial structure could be attributed to that look. Plus, the lone human they had with, Mary, was an oriental. All of this evidence leads me to believe that the Kromaggs from "Invasion" were the Oriental equivalent of Kromaggs.
The new Kromaggs we now see mostly would be clasified as a European look...in fact, one could speculate German (thus adding some credence to why an alternate earth Hitler would so quickly take in the Maggs before him).
This cultural difference could also explain the "language rule" given in "Invasion". In that episode, the Kromaggs stated through Mary that they would not degrade themselves to speak a human language...yet, the new Kromaggs have no qualms. This is readily similar to cultural differences of our world...Orientals did not even have contact with the outside world for many centuries and refused to speak to anyone not of their race. While on the other side, peoples of Europe have never really had any problems communicating...they just often had a problem liking what each other said. Of course, once the humans developed the weapon on Kromagg Prime, the Kromaggs all banded together with a new common enemy. Humanity...and especially Michael Mallory.
A second theory to explain the change is that the Kromaggs can appear in any form they choose through their mental powers. This theory says that the Kromaggs were taking up a more fearsome appearance in "Invasion" to intimidate, but have now chosen a more human look for some undisclosed reason. The problem here is that there is no real proof, and it seems like a "cop out" to use that explanation. Of course, if the theory is true...we can be sure that the current form is the true one after the fourth season episode "The Dying Fields". After all, Hu-Maggs would not yet have mastered the ability of altering appearance (if they can even do it at all). With that in mind, their similarities to the Kromaggs we now see have to be genuine and not illusion.