The precautions necessary when upgrading Plone itself are similar to those used when upgrading products. An overview of Plone's release process and history can also be informative when contemplating an upgrade:
Plone's general version-numbering philosophy has been inconsistent over the life of the project, but it is currently as follows: a major release, such as 3.0 or 4.0, may unapologetically break backward compatibility with some third-party products, though migration paths for content (at least out-of-the-box sorts like news items and pages) are always included. Tenths- and hundredths-place releases (like 3.2 or 3.2.1, respectively), are mostly backward compatible with regard to add-ons. However, the complexity of the system and the lack of formal guidelines on what add-ons may and may not touch make backward compatibility less than one hundred percent. There are always some add-ons that break in a tenths-place release and a few unlucky ones for each hundredths-place release...