You can always put your world seed through the World generator and compare between Pre and post Mistalnd versions.

But based on what you said you'll loose the fun of discovering the Mistalnds on your established world.

I have an older world myself and must say the "Tiny Islands of mistalnds" never happened ot me.

here is my OLD world seed Pre mistlands:
https://valheim-map.world/index.html?seed=DSbDytX5eY&offset=0%2C0&zoom=0.600&view=0&ver=0.207.20

And her is the same seed post mistlands: https://valheim-map.world/index.html?seed=DSbDytX5eY&offset=0%2C0&zoom=0.600&view=0&ver=0.212.7

You compare and see if the slight differences look worrying to you as I have to assume your world would change in a similar way.