Smooth upgrade experience
One of the central tenets of Plone 4 is to maintain backwards compatibility whenever possible, while still cleaning up the infrastructure and improving the code base and its performance.
We've worked hard to preserve backwards compatibility for add-on products and customizations. Most current add-on products will “just work,” while others will require only minor changes. Feedback from our alpha and beta users point to the easiest, smoothest Plone upgrade experience ever, and we're excited to have so many Plone 3 sites be able to upgrade with a minimal time investment.
We ship Plone 4 with a well-tested upgrade script that includes a "dry-run" test mode, so you can see if the upgrade will be successful without making any actual changes to the database. Of course, backups are essential as part of any upgrade, but those are always a good idea for lots of other reasons too. :)
It was hard work, but we believe we have succeeded in making Plone 4 the easiest release to upgrade to yet!