A magic shop owner checked his shelves, making sure every item was in the right spot. Customers kept putting things in the wrong places. Then he noticed that on one shelf, something was seriously wrong. One of the items could make physical changes to every person in the world. Selling that was seriously bad--someone could use it and transform the whole world, including him, the shop owner, and a lot of other people thousands of miles away who don't want some guy on the other side of the country transforming them by remote control. He picked up the object, and pointed his wand at it, zapping it into a pile of ashes, which blew away.
He moved one of the other boxes to the front to take up the empty space. This one should be safe to sell without getting in trouble. It was a version of the other item that only affected one person at a time.
The device could alter a single person's sexual characteristics and gender. It was line of sight, but you didn't need to do something obvious like point it at someone. It had a handy diagram of a body and even allowed you to zoom in, so you could specify what parts of the body changed to their opposite gender version. Internal organs change to match external ones, so someone with a vagina can get pregnant. It couldn't alter traits within a gender, such as changing breast size--if you wanted to give someone breasts, they got whatever size breasts the device thinks naturally go with a female version of them.
The device let you choose whether people remember the changes or whether reality is altered. The user of the device always remembers.
The device only makes physical changes. If you change reality, there could be minor mental effects as a result of that. For instance, if you turned a typical man into a woman, she'd be attracted to women, hate wearing dresses, and like to do the same masculine things she always did. But she'd remember growing up wearing bras, so she wouldn't mind wearing bras. If she's a doctor, she'd probably still be one, but being a football player might not be possible. And a lifetime of growing up with a female body and hormones _might_ push her towards being attracted to men a little bit more than otherwise. Maybe.
Changing reality also changes sexual identity enough that the targets don't usually have dysphoria--if you change a man into a woman, and change reality, you don't get a woman who desperately wanted to be a man all her life. And a man partially changed to have had breasts all his life isn't horrified at having them--though he may be upset about how people treat him for having them.