So I created some old worlds in 1.17 and when the 1.18 update came, i made a copy of that world, then i went into that copy and selected the button “make infinite”, as i explored the world i noticed old chunks i never walked i  had been changed, however there was no blending, there was also no blending between completely new chunks (where the border used to be) and chunks i had been in.

however, that is insignificant compared to the other effect I noticed, the build limit was still from y= 0 to y = 256, and the bedrock and space underneath was not updated to have the new generation.

I have attached images of those as well as the world file as a zip