Can I change Event Types in bulk?
If the events are all part of a repeating event series, then you should be able to change them by changing one, and selecting the option to change all related events.

Otherwise, I'm afraid there's no built-in way to bulk change event types.  If you know what you're doing with the API Explorer or MySQL, you can bulk change the event type on the back end - there's no real harm unless you have custom fields tied to the old event type, in which case they might show up unexpectedly on a report.