In Problems installing (M)ELPA packages Sebastien wrote:
While using `package.el’ fulfills a long-awaited dream for easily adding and upgrading Emacs libraries, I now feel upgrading packages is much more complex than what I anticipated.
At least two of the packages I use daily (Org and Helm) require to be installed from inside a fresh Emacs session (emacs -Q), where nothing has been loaded.
I had wondered but never found out why one must follow this approach when installing org from scratch. Good to know!