You need to update the overworld before upgrading to 1.8! Please see https://docs.volmit.com/iris/engine/updateoverworld

• Overhauled the Caving & Carving systems
• Caves now generate with worms (in the same way vanilla generates caves), this means they have dead ends and look more natural!
• Per cave specific biomes! This means each cave has it's own biome & transitions to different cave biomes much more cleanly.
• Fully custom ravines! Ravines are defined by the dimension starting with a worm path and a depth / angle. Ravines can also fork caves or even more ravines off of them
• You can specify the "wall" part of caves & ravines along with the floor and ceiling as usual.
• Ravines & caves now support submerged bodies. This means that ravines under an ocean (or caves) are fully submerged up to the fluid height that they originated at!
• Added a new experimental snippet system in Iris. You can essentially swap any component of json into it's own separate file & reference it in multiple iris configuration files. We will document & improve this feature over time as it is currently a beta feature
• Added configuration options for console / ingame custom colors
• Cave / ravine openings no longer have tons of floating grass / flower blocks, water blocks update near cave openings & other block update fixes such as these
• Iris has a brand new command system, with colors, tooltips & tabulation! Hopefully this will make using Iris a bit easier to interact with!

We're not done with carving! The entire 1.8.X set of versions is scheduled to have more features with carving such as rivers, lakes, overhangs & more! Stay tuned!