People have asked what properties are required for schema.org vocabularies, but (afaik) there’s been no official answer. Here’s what I’ve discovered from the Google Rich Snippets Testing Tool (← link to test file), the closest thing we have to guidance on implementing schema.org vocabularies. Note the tool’s warnings may or may not be according to schema.org vocabularies, as none of this is actually defined (sigh). You could also try referring to Rich Snippets documentation or asking on the mailing list.

• you must include at least one property from the vocabulary you’re using
• Recipe, Person and Product require the name property
• Event requires the name and dtstart properties (and that the event’s url property be on the same domain as the base url)
• MusicRecording requires the name, url, duration, and inAlbum properties
• values are only sometimes verified: for example Event’s startdate and enddate plus MusicRecording’s duration are validated against ISO 8601, but Recipe’s cooktime and totaltime aren’t
• sometimes the tool gives incorrect warnings, for example warning about Event’s “dtstart“ and “dtend” (not startdate and enddate)

Not many properties are flagged as required. This means if you’re changing from <time pubdate> to BlogPosting’s datepublished, (amazingly) you only need to declare the vocabulary and this single property. At least until the vocabulary is updated :)