Will Sturdy still prevent Ogerpon-Cornerstone from being OHKOed after Terastillization and Embody Aspect becoming the ability? Will Ogerpon-Wellspring still be immune to water attacks? Will Ogerpon-Hearthflame still be able to bypass opponents’ abilities?
Nope. Embody Aspect is not like As One.
Upon Terastallizing, Ogerpon's Ability will change to Embody Aspect, and one of its stats is raised, depending on the mask:
Me and CinderClod were able to test this in this replay. Yes, Embody Aspect replaces the prior ability.
Hope I helped!
Source