i don't think that the 4th GF planet

I assume that the change is due to uniqueness. While the other planets already had their thing, a planet which is based on construction, does not have the same flavour as the others so far, nor does it seem as suitable as a place of conflict.

I agree that Guardshell and Demolisher have a better place with the Gigalonia construction cybertrons/destrons then with earth. I also think it likely that both end up with a new force chip once there.

If it's indeed a late in change from construction to giant, that would explain why that on that planet there are the giant transformers and the minicons, yet the giants are not headmasters or motormasters or the like, which would have been more obvious in my view if the three giants were actually bodies for minicons.