Upgrading ExpressionEngine to 2.7.3
I just recently upgraded ExpressionEngine (EE) from version 2.5.5 to 2.7.3. It took multiple attempts to get it right. There were a string of PHP errors after upgrading, some due to an incompatible third party plugin. The major issues had to do with the newly reconstructed relationships. EE relationships changed with version 2.6, and the new format is incompatible with the old format. If you used relationships in your templates in the past (I did), your template code needs to be updated when you upgrade. Conveniently, the upgrade routine alters existing templates to use the new relationship format. Even so, I still had a number of PHP errors after upgrading.