How to handle those column conversions is high on my list of things to improve. For now, you have a couple options:
You could use changelog parameters. Define parameters like “${char_type}” and in your changeset set
<Another option would be to create an extension that overrides the liquibase.datatype.core.CharType and other classes. You extend the base classes then override the toDatabaseDataType() method to return whatever works best for you. See liquibase.org/extensions for more information on the extension system.
Nathan