Context:

I had an Old Type world (created around 1.16 and last loaded 1.17.41) that I wanted to continue playing on as an expanded world in 1.18. After updating the game, I made a copy of the world (without opening it), made said copy Infinite, and opened it. I wasn’t given the new “World Update” screen (that lets you know that the world will be changed and to create a backup). Once I began exploring outside the expanded world, I noticed that all of the generation was 1.17 terrain, and even had things like the now discontinued “lakes”. It never became 1.18 terrain.

I made another copy of the original Old Type world. I opened the world, exited and then made it Infinite. There was a slight difference this time: still only 1.17 terrain, but there was no biome blending from the existing Old Type terrain to the outer new chunks (all of the world’s borders had biome blending the first time), except for one corner of the Old Type area which did have blending, probably because it wasn’t in simulation distance when I opened the world in 1.18.

Another detail is that, before the 1.18 release, I made an expanded Infinite copy of the world in 1.17.41 and explored all of the area that bordered the original part of the world. I tried to open this version of the world and was given the “World Update” screen finally, unlike the other times with the two expanded versions. I proceeded with opening it and the world seemed normal, having 1.18 generation and biome blending, where I haven’t already loaded 1.17 chunks.

Now Here’s an Abbreviation/How to Recreate:{}

Create Old Type world on 1.17.41 (or recent-ish version and then load it up to 1.17.41)

Old World last loaded in 1.17.41, and expand then opened on 1.18 gets:

• World Update screen doesn’t appear, not supposed to happen
• Old borders have biome blending after expansion
• Only 1.17 type generation format on new chunks, not supposed to happen

Old World last loaded, expanded then opened on 1.18 gets:

• World Update screen doesn’t appear, not supposed to happen
• Old borders don’t have biome blending after expansion except for parts of the border out of distance when world was loaded in 1.18 before being expanded
• Only 1.17 type generation format on new chunks, not supposed to happen

Old World last loaded, expanded then loaded again on 1.17.41, and opened on 1.18 gets:

• World Update screen appears as intended
• Biome blending is on borders of unexplored chunks
• 1.18 generation on new chunks as intended

Thank you very much for reading through this long report, and working through the many bug reports from 1.18. I hope I only gave necessary details. I’m not sure if the biome blending differences is part of the bug I’m reporting, just felt it may be useful to know. Also, all of the snapshots included were taken from my switch on 1.18, and examples of the three versions of the now Infinite world in order from 2nd, 1st, 4th, 3rd) of how I mentioned them before, except the 1st, which is just the unexpanded Old 1.18 world