The Mundaka Upanishad (3.2.9) makes a bold statement, “Anyone who knows that supreme Brahman becomes Brahman indeed.”  This statement gives rise to two potential doubts:
The Issue of Action: The verb “becomes” implies a transformation or action. However, according to Advaita Vedanta, knowledge alone—not action—is the direct means to liberation.
The Issue of Essential Identity: The mantra might suggest that an ignorant person is not Brahman, whereas the fundamental premise of Advaita is the intrinsic identity of the Jiva and Brahman.

Resolution
At empirical level, knowing an object does not cause the subject to become that object—knowing a table does not make one a table. “Knowing” and “becoming” can be identical under only one condition: when the knower is already identical with the object to be known.
As Shankaracharya comments in the introduction to Taittiriya Upanishad (2.1.1):
“And hence liberation is not an achievable result. A traveller has to reach a destination different from himself. It is not as though the very place non-different from oneself can be reached by oneself.”
The mantra addresses a situation where the subject and the object are essentially one, yet due to an obstacle, the subject fails to recognize this identity. That obstacle is ignorance, and its sole remedy is knowledge. Upon gaining knowledge, one realizes Brahman as one’s very Self.
Clarifying this verse, Shankaracharya writes in his commentary on Mundaka Upanishad (3.2.9):
“He who knows that supreme Brahman directly as ‘I am verily Brahman’ does not follow any other course. In the matter of his attaining Brahman, even the gods cannot raise any obstacle; for he becomes their Self.”

Thus, the knower “becomes” Brahman only in the sense that he recognizes his true nature, not in the sense of acquiring a new status or undergoing a modification.

The resolution of the second doubt follows directly from this understanding: every Jiva is inherently Brahman. An ignorant person simply remains unaware of this fact. Upon gaining self-knowledge, the individual does not turn into Brahman, rather realizes that he has always been Brahman.

Note: Extracts from Eight Upanishads with Commentary of Shankaracharya, Swami Gambhirananda, Advaita Ashrama