Technically, it is legal, but a lot of cities and counties refuse to acknowledge it even though not acknowledging it is legal.
A friend of mine was able to get his gender changed pre-op AND pre-hormone treatment, and marry his wife with just a note in St. Petersburg.
Most of what I’ve found, though, is that many places will still ignore it. In some places, a birth certificate change is required first, in some, it’s not, but in many places, a note is all that’s required, which is amazing
You can find a discussion of this discrepancy here.
Here’s another article that discusses this policy.
Oops, accidentally posted this to my personal blog first.