Version v0.40 of Metabase is no longer supported. Check out the docs for the current stable version, Metabase v0.63.
When should I migrate from H2 to MySQL or Postgres?
As soon as you plan to use Metabase for anything other than testing: H2 is neither as fast or as robust as production database systems. The migration is fairly simple; please see this guide for instructions.
Read docs for other versions of Metabase.