How to migrate (painless) Redmine instance from PostgreSQL to MySQL

Like in topic, I've working instance of Redmine, witch i can shut down for only little time period, how can I migrate my projects etc. to MySQL backend from PostgreSQL?

https://github.com/adamwiggins/yaml_db

This allows you to export any database used with rails to a yaml file and import it into any other database.

So,
PostgreSQL -> yaml -> MySQL

I personally had used taps (https://github.com/ricardochimal/taps) with great success. It allows online migration over the network between about any relation database used in the wild.