I had some out of memory errors whenever I tried to upgrade my WordPress installation for this site. Finally someone clued me in to the fix. Deactivate all your plugins first, then do the upgrade. When you are finished upgrading, reactivate the plugins. It’s a simple memory management issue, WordPress tries to old all the plugins in memory and then has no room for doing the upgrade.
Anyway, the upgrade proceeded normally after I learned that trick. Now you know it too.