As Queen you are desperate for the help from a highly skilled wizard or sorceress. Her kingdom is crumbling as the neighboring kingdom has a king in possession of a talisman which grows stronger with both time and use in the form of spilling human blood. Thus far unable to stop the death from disease, natural disasters, rising levels of violence brought on by temporary insanity, then how can she hope to stop the damage now when it is more powerful than ever and growing stronger. The leader of this struggling nation stumbles upon the name of the individual who was powerful enough to create the stone but whose identity had remained hidden due to the fact that the talisman as it is today is a tool of misery and destruction where at the time of it's creation was a tool to aid in health and well being of all and when needed to aid in the defense of those in the nation now being preyed upon.
The only problem is that the sorceress who created it is young and vibrant in all ways accept for her slipping memory. Shhe may be capable of creating some of the most powerful, clever, and self sustaining spells but she often gets confused. Her youthful appearance and vibrancy are a result of magic she used on herself to stave of the ageing process. Occasional confusion may not seem like a big deal but when you are wielding such potent magic leaving out a part of the process needed for it to come to be can lead to disastrous, and irreversible mistakes.
If we go fxf I would be a sorceress and you would do a few posts as the king contacting the sorceress in search of her aid. It takes little time at all for the sorceress to neutralize the talisman but one part of the spell pertaining to the protection of the nation led by the King is either left out or added when it should not have been turning the King into a female. She is indignant and demands to be turned back to her former body as a male. He never stops trying to fix it but at one point he accidentally has placed another spell on him now a her as she is hit with a love spell falling for the sorceress. She would reverse it if she could but is not at all bothered by the very attractive queen he created. She begs to be allowed to stay female as the female orgasm blows the male orgasm away and is like comparing a wave to a tsunami. We can discuss where it goes from there.
If it is mxf then I would be the Queen turned into a King and falling under the same spell as in the fxf example and my partner would be the sorceress who tries to reverse it but also finds the new King she created quite attractive. He then makes one demand of her to rectify the mistake she has made. "You shall place a spell upon yourself which forces you to please me in every way I desire." Appalled at the prospect of turning herself into what would be essentially his sex slave she resists but the very forgetfulness that created the errors so far becomes responsible for fulfilling his request when she mixes up spells and accidentally turns herself into his sex slave.