In Age of Wonders 4, Major Transformations can significantly change the appearance and abilities of your ruler and units. But if you prefer to keep your ruler’s original appearance, you can indeed turn off Major Transformations.
As pointed out in a PSA by a reddit user TheItalian567 in this post.
For example, if you’re playing as Vampires and don’t want your ruler to transform into a bald, ghoulish creature, you can disable this feature in the game settings.
However, be aware that turning off Major Transformations also prevents your ruler from gaining certain unit tags and benefits that come along with the transformation. Using the Vampire example, if Major Transformations are disabled, your ruler won’t become undead and therefore won’t benefit from the synergies that the Wightborn transformation brings.
Related: How City Expansion works in Age of Wonder 4
In summary, while you can disable Major Transformations to maintain the original appearance of your ruler, it’s a trade-off as you’ll miss out on some of the benefits that these transformations bring to your gameplay.