I'm not sure why nobody has reported it but I can probably be helpful.
Basically, with the plugin migrating to custom post types you are migrating the actual event to a new permalink structure. Because of this it will have a new URL.
The problem is that without a redirect to the new URL Google and the search engines will drop the content from their indexes as '404 unknown' and reevaluate the ranking process negatively.
Even if creating the content elsewhere its a good idea to let the search engines know that content has moved. If the events were to keep the same urls as the original posts (before migrating) then this would be of no consequence and 301 rediects would not be needed.