You can still download the original KML files for your custom maps until June 1st, just in case they weren't properly converted. To export a map, go to My Maps, click "open a map", pick the map you want to export and click "Download classic My Maps data" in the settings menu. If this feature is not available, then the map was created using the new My Maps. There's also an option to "export to KML", which downloads the current map.
"If you aren't happy with the way that your maps upgraded, you can download the original, pre-upgrade version of your maps as KML files from within the new My Maps. These files will be available only until June 1st, 2015. After June 1st, the pre-upgrade version of the KML files will be removed," informs Google.