I don't think Kui quite tries to implement "video game mechanics" in Dungeon Meshi, rather it's just some similarities from the fantasy genre (there's the stats from the AB but I mean in the main story). For example something interesting is that she doesn't really use the word "mana" (which is common concept in video games) rather she uses "magic" or "magic power" mana comes from the translation which probably wanted to keep it more similar to "video game mechanics in anime" which is a trope used in lots of fantasy anime (especially isekais I think). It also happened with translating "tallmen" as "human" and "half-foots" as "halflings" (brazilian translation still calls tallmen by humans), which I think it's a shame since Kui is crafted an unique fantasy world so adapting it into a generic "video games in anime" is sad.
That aside - That's an interesting theory, I wonder if anyone would want to risk going thru that to be able to live as another race? In the manga they have questions about the transformation like if the change would affect their lifespan, I wonder how "good" of a transformation it is. Since it's not meant to last it could be the case that there would strange side effects to the transformation (cause they don't really seem to fully assimilate the genetics of the other races, for example there's no canon full elves with beards or dark hair but elf senshi has both, so there might be other things about him that are different from other elves)
I don't know enough about fungi to say if something like that could work tho, and even in the stage they are everyone who gets transformed by the changelings are already spreading spores and if they make a ring they transform other people, they would probably have to make that stop too to make it safe.
The thing about "eating to make it part of you" might not work for something like changelings tho, it's their magic that transform others while they don't transform themselves, so even if you used magic to somehow assimilate the spores into yourself I don't think it would have that effect?
I also think it would probably get weird fast if changing your race was accessible, especially with the vastly different lifespan and the fact long lived races have way more privileges in that world
I do think it's something cool to theorize about tho, these are just my thoughts!