With 1.18 coming out a few days ago I want to address what will happen with the update in regards to the Mr. Green SMP server. It has been decided that there will NOT be a world reset when the server updates, for several reasons:
An old world could become a feature of the server. Old worlds contain a lot of history about players and events throughout the lifetime of the server.
There are several people who would like a fresh start on the server, but these people can still just leave their items at spawn and travel away to fresh chunks to achieve such a fresh start. With a world reset everyone would be forced to start over, which is not something a lot of the playerbase wants. With this in mind, not doing a world reset should still more or less accommodate people's different needs.
Newly generated chunks will have the full terrain features of the 1.18 update, and finding such chunks is not too far away from spawn. Such areas can be identified as a black area on dynmap. 1.18 has a terrain-blending feature which ensures the transition from <1.17 generated chunks to 1.18 generated chunks is more or less smooth.
Chunks which were already generated in 1.17 or earlier will remain the same, except for y level below 0 to -64 which will have the new cave generation. This means the new cave generation will be available for the entire world including already generated chunks.
Currently we are waiting for our plugins to update before the server version can be bumped. I expect this to happen within a few days.