Here's the Endocrine Society on sexual dimorphism:
' Sex is dichotomous, with sex determination in the fertilized zygote stemming from unequal expression of sex chromosomal genes. By contrast, gender includes perception of the individual as male, female, or other, both by the individual and by society; both humans and animals have sex, but only humans have gender'
'sex is not the same thing as gender and using these terms as equivalents obfuscates differences that are real and important in society in general and biomedical research in particular.'
'Sex is an essential part of vertebrate biology, but gender is a human phenomenon; sex often influences gender, but gender cannot influence sex. '
academic.oup.com/edrv/article/42/3/219/6159361
I don't see anything in there specifically explaining that it's not possible to change sex, but I think that's because most people wouldn't even imagine it's an argument that needs to be made.
It's not possible to change sex. Ever.
Gender is infinite, variable, subjective and changeable - sex is dichotomous and fixed.