Today I upgraded the blog from Plone 2.5 to Plone 4, via quintagroup transmogrifier and XML export and then import. It went very well, but: A side effect of that was that although the effective dates were preserved, the modified date seems to have been set to today, the time of the import. On all items. I use basesyndication's templates for the feeds and they have a piece for the last updated field.
My feed should now be empty (save this blog post) and I hope the old posts will clear out from planet.plone. Sorry!
