Like you, I understand that plugin_wiki will eventually be deprecated, but you can still use it as it is and I am sure Massimo will accept patches and fix bugs etc. However, I can't imagine there would ever be any kind of 'conversion' program.
The basic design of auth_wiki is better and integrated, and it is up to us to suggest how it can be improved. Let's put it this way, what would you like to do with auth_wiki that you currently cannot? Regards, D --