How 18-inch guns would reshape the Admiral class

Would an 18-inch-gunned Admiral class undergo the same development changes as HMS Hood?

Not exactly; designing for 18-inch combat would require a heavier protection baseline, so later post-Jutland changes would start from a different ship.

A Hood designed around 18-inch guns from the outset would not simply be the historical ship with a larger main battery. British designers would also consider the possibility of fighting an enemy armed with 18-inch weapons, pushing the protection scheme beyond one sized for 15-inch opponents. Belt or other armor might move toward the 15- or 16-inch range because Britain could manufacture and proof heavier plates. Post-Jutland changes would still occur, but their form and scale would differ because the baseline ship was already larger and better protected.