In Cryptum, the Primordial is described to have eyes as well as arms. Then in Primordium, it changed quite drastically in its appearance, being much more curled up like a spider. The question, was the Primordial a Gravemind the entire time and had somehow changed its shape?
Yes. The Primordial was the first (known?) Gravemind. The ones that have formed since are basically just reincarnations.
While the primordial was the first gravemind the first time it was encountered seems to be it before it changed form as one form speaks of it having a scorpion like tail coming from the base of its neck with four arms around its abdomen and the second time it is seen its lost the barb gained many arms and appears sickly or grotesque. Whatever actually happend it certainly appears that it changed forms between its first sighting and its second.