As I mentioned in the expanded version of the sahuagen spells post, Otherverse's The Modern Grimoire has two spells that allow for the transformation of one species into another. Fostering is used on infants either freely given or taken in war (it fails on others). Transfigure Species transforms any one race into another selected by the caster. The most important difference is though they both change appearance, Transfigure Species makes a complete change, whereas Fostering effectively makes the infant and resulting character a half breed, gaining the bonuses and penalties of both races. These are powerful spells with some serious implications that a GM should consider before using them. If they exist, how might those cultures that have them react?

Monster cults. Some parents would want to have their children turned into dragons (or nagas in an Indian based setting) or other long lived and very powerful monsters. They would do whatever the monster with the spell asks. And the results could make an interesting patchwork within that society. If a family can pay only to have one or a few of their children changed rather than all of them, some people would have monstrous siblings.

Racial purity lashbacks. Other people are going to find the idea so repugnant that they will react. It may be with emigration, violence or shunning. Society will break up into two or more fragments. What is worse is that it might feed on itself, make those people more extreme, rejecting even their former allies of other races. Dwarves may have nothing to do with humans being turned into ogre mages but they bare the brunt as they are weaker than the oni and make good scapegoats to the minds of those who hate the ogres.

Child theft. Some peoples that have the spell may raid others not just for their wealth, but their children. The human supremists may take the idea and decide that dwarven children would have a better life as humans. Entire species may be eradicated by the slaughter or conversion of adults and conversion of children. This may be driven by society or religion. If the gods get involved, conflicts are going to rage until the stars go out.

And it may get very strange depending on what races can be affected. If an illithid can be transfigured into a human, do they still have a taste for brains? Would they hide from the sun? Or would they go in the other direction, attempting to clear away clouds to enjoy the sun while educating others on the dangers of the deeps? In some editions dolphins are more intelligent than baseline humans. Would that make them a race that could be converted? How about using Awaken on rodents? People made from giant rats to replace those slain by a plague or used as disposable troops in a never ending war?

As written, Fostering is for clerics and Transfigure Species is for alchemists, clerics, druids and sorcerers/wizards. Changing the spell lists for both can also have some implications. Those with divine backing will have different uses than those who answer to themselves (or at least gain their power through their work rather than a divine battery).

I think both spells are great and am considering using them in Oathbound as vivimancer spells. But I have no illusions that they will bring conflict, much more conflict than most spells. That is why I decided on the tar baby as a representative picture.