Starting at 6th level, when you cast an illusion spell that has a duration of 1 minute or longer, you can use your action to change the nature of that illusion (using the spell’s normal parameters for the illusion), provided that you can see the illusion.
Relevant snippet from the Major Image spell:
Physical interaction with the image reveals it to be an Illusion, because things can pass through it. A creature that uses its action to examine the image can determine that it is an Illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the Illusion for what it is, the creature can see through the image, and its other sensory qualities become faint to the creature.
If I cast a major image of a cage, and the enemy sees through it with a successful check, can I use malleable illusions to change the image to a frightening beast charging out from the trees? Or will the fact that he's discerned that the illusion is not real mean that anything I change the illusion into will automatically be see-through and faint?