Cisgender is when your sex and gender identity match.

Transgender is when your sex and gender identity don't match.

If someone is transgender, we have to change one of the two things to correct the mismatch between gender and sex for them to stop experiencing symptoms of gender dysphoria.

That means we either need to change our gender identity, which isn't currently medically possible and probably isn't all that ethical, or our sex, which is fairly easy to do by switching our dominant sex hormone.

When we change a person's sex to match their gender there's no longer a mismatch, so you no longer meet the definition of transgender.

You could still say you were born transgender, or you could use transsexual to indicate that your sex is different to the one you were assigned at birth. Or you could ignore gender identity and say that because society originally thought you were the wrong gender and had to be corrected, that you'd changed your gender socially and therefore will always be transgender.