Keep your tmux sessions although client has upgraded
I started an upgrade of a machine and decided to do it over tmux so that losing the SSH connection would be no biggie. However when I came back I wanted to reattach to the tmux session and couldn't; the client had been upgraded so it wouldn't connect to the old sessions. Then you can do like this:
pgrep tmux
<process id>
/proc/<process id>/exe attach
Pretty awesome hack, if you need your tmux working and not want to lose all your sessions: