I tried upgrading from 2.38 to the latest version (2.39.6) through the WP admin panel. When I activated the new version all of my maps, mashups etc completely disappeared. The Mappress field in each post that had previously had a map was blank.
I deactivated it and reactivated 2.38 so the site is fine for now. But how to I upgrade without all of the data going missing?
Just a follow up to say I waited for 2.4 to try again. Upgrading works fine for 2.4, not sure what happened with 2.39.6 (yes all upgrade steps followed appropriately).