Yes. It is possible. It's based off of the same concept as the worker, leader, and army units changing graphics. (Though I prefer the term "Redress" the unit, for they are not changing into a different unit, just different appearances.) This is not really realistic or reasonable, since if you were to advance to the modern age, and hypothetically an enemy is in the ancient age, then you'd have "spearmen" with guns vs their "spearmen" with spears. Only Outposts, Airports, Colonies, Forts, and Barricades can do this.