How to upgrade from 3.0 and keep saved sessions ?

[oglops]

4.0 changelog says

(breaking change: you have to re-save to be able to properly restore!)

But how do I migrate to 4.0 ? I suppose I need to load 3.0 sessions, and with tmux running, on the fly update to 4.0, then save the session ? I tried run-shell the 4.0 .tmux file, and save, but it did not work, When I restore using 4.0 plugin, it crashes the server.