Do you have to be a transgender to get a sex change?

No. In fact, it is best if you are not "transgender." Sex reassignment is for people born with transsexualism, not people merely manifesting some form of "transgenderism." Many, if not most, transgender people love their original genitals and use them sexually. Instead, you had better be born with the condition of transsexualism. Transsexualism is not something you do or become, but something you use hormones and surgery to get rid of.

Now, there are people who don't actually have transsexualism who get the surgery. Many TGs are not honest with themselves and get the TS-only procedure of SRS by mistake. They may feel a need to compete with actual TS women who need the surgery. But then there are the TG types and even gay men who know they don't have transsexualism and who have stable personal reasons for wanting the surgery.

The main thing the surgeons look at is if the person meets the standards of care. And therapists these days are flexible enough to approve even nonstandard types of patients as long as they are confident the surgery will do more good than harm.