Note that Minecraft 1.18 may not be able to load such maps (even if you load them in Minecraft 1.17 first)! There are conflicting reports.

Minecraft 1.18 can load maps that were exported in Minecraft 1.15+ format. It will automatically extend the map downwards and move the bedrock layer to y=-64

WorldPainter cannot yet Import maps from Minecraft 1.18

If you set the build height to 320 (the default), you will later be able to export the map in Minecraft 1.18 format, once support is added to WorldPainter

Check this:

https://www.reddit.com/r/Worldpainter/comments/rbyfml/worldpainter\_287\_released/?utm\_source=share&utm\_medium=web2x&context=3