Some people think that at the end of the game, when you absorb the SA-X in it's parasitic form, you become the SA-X itself. Well folks, you don't. Use some logic.
When Samus absorbed the SA-X, she killed it, as opposed to changing into it. The SA-X is a parasitic being, an X parasite. Samus is part human, part Chozo, and part Metroid. Because she is a Metroid, she kills whatever X she absorbs.
Now, this rumor how you morph into the SA-X probably comes from the color change of your Gravity Suit after absorption. That happens because the SA-X has so much DNA and data to hold and keep straight, that it promtly infiltrated everthing- it reprogrammed Screw Attack, Space Jump, Speed Booster, High Jump, JumpBall, Power Beam, Spazer Laser Beam, Wave Beam, Morph Ball, Bomb, Power Bomb, Plasma Beam, Power Suit, and Varia suit. The Metroid half of Samus killed the X and applied the data to her Suit.