Ed has gone on record as saying that there are plenty of queer characters in his Realms, and that editorial has prevented him from publishing anywhere near as many as he would like. That includes trans characters. Ed and the other long-time Realms writers have snuck in what they can. For example, Eric Boyd hid a trans character in Faiths and Pantheons.
We don't have details of transitioning in the Realms. We know it's a thing, and we know it takes some time from the above example, but we don't know how it happens. Neither the belt (a puerile transmisogynistic joke that isn't actually from the Realms) nor Elminster's time as Elmara (specifically echoing the myth of Tiresias) are representative of actual trans lives.
Personally, as a trans person, I think it's something a deity fixes for their worshippers. We know that deities do take steps to protect the health of their church, and deific intervention in people's lives is rare but not unheard of. I don't think you talk to a priest, you just pray to your patron deity hard enough, and it happens.
No matter what the answer is, the character in SoD is perfectly fine and canonically appropriate. It's an example of good representation and a good addition to the series.
