that sounds like a really dumb explanation and considering Ben knows his aliens by heart and would most likely not how he got Terraspin, I seriously doubt that they're the same. Plus, their powers are different: Terraspin can create a lot of wind, Shellhead can just hide inside his shell (which is why he was given to Kenny, he served for defense, not attack like Terraspin does)
Shellhead can also be a fusion, the word "head" is also in Diamondhead. I don't think that Ben can forgot the name "Terraspin" if he would forget, then why would he name again? He can ask from Gwen or any other character😕 Wait! Even if Ben 10k forgets the name Terrapin, his Omnitrix would never forgot, Omnitrix knows all the names of Ben's aliens, as we saw in the War of the World: Part 2 that Ben asked Omnitrix to transform into Humungousaur and Omnitrix transformed him into Humungousaur. So, Omnitrix is obviously knowing the name of all aliens, then why would he rename as he can know by his Omnitrix?