How do you replace entities seemlessly?

Hey this is my first time modding and I want to create a mob that changes model after it spots the player.

I've tried looking around and the only post that seems like it could help is years old and won't load. 

I've tried despawning the initial entity and spawning the second phase in the same location with a procedure but it doesn't seem to work and the world freezes instead. I've also tried spawning the second phase by itself and that seems fine, any help would be appreciated dearly.

https://youtu.be/zmL97ovp0kU?si=-dlOGbx8OoHVCjHY

From what I can see this mod appears to do something similar with it's entities changing states but does anyone know how to keep the original mobs directions as well?