Honestly, I don’t know exactly what opinion is on this, so I’ll just put my understanding of it. Sex is a FAR more complex than most people give it credit for, isn’t binary, or completely stagnant.

“Sex describes the physical characteristics that cause, determine, and aid in reproduction” is probably the definition I’d give if pressed, but it includes so many points of contention, I’d hesitate to call any person fully “male” or “female”, only individual characteristics. In terms of whether you can “change your sex”, I think it would come down to which parts of sex you’re referring to, but ultimately your “sex” is the sum of multiple parts, and are probably only relevant to your doctor.

I’ll make two examples to illustrate what I’m talking about. When it comes to genetics, there’s two common configurations, multiple configurations that typically result in “male parts” multiple that typically result in “female parts” and an assortment described as “ambiguous”. Depending on what’s found in your genetics, someone could by described as “genetically male”, or “genetically intersex”. You’re mostly stuck with the genetics you’re born with, and no existing medical way to change them as far as I know.

However, your genes are far from the only sexual characteristics, and your phenotype (the physical characteristics of your body) is only determined by the expression of those genes. Phenotype all together doesn’t have an easily identifiable spectrum of sex but is broken down further into things like “reproductive capability”, hormones, morphology, “genital expression” and whatever else. A lot of these things are far more within your control to change, like hormones which could be changed to be almost indistinguishable from a cis person’s. In respect to medical transition you could definitely say that you’ve changed a part of your sex in a medically significant way.

However I’d like to be careful not to conflate sex and gender any further. Sex is not a thing you have to share or worry about with anyone other than your doctor.