Migrate existing saves to new world genenerator settings.
The migration is done in migration script, be patient when stuck at loading save with full progress bar.
After the migration is done this mod can be safely removed.

To apply migration again a save has to be made with the mod removed or deactivated.

I recommend setting map settings before enabling this mod with https://mods.factorio.com/mod/ChangeMapSettings

Features:

• updates tiles to new biomes
• retains existing lakes
• retains player placed floor, including modded ones
• option to wipe or recreate decoratives like grass and small stones
  floor tiles and railway tracks are kept clean of decoratives
• option to recreate trees, large rocks and cliffs
  recreated entities will not be spawned if they would collide with player built structures
• option to limit how close to player structures new trees and large rocks are spawned
• option to keep existing cliffs while adding new ones (results may look a bit extreme)

Known Issues:

• Using migration script doesn't allow unloading clusters from the temporary surface resulting in large memory requirement ~2-3 times save size.

Credits:

staviq - NiceFill for the method of copying tiles from a hidden surface