The method would be "api enlightenment" after which you will have the extra "enlightenment" option(s) on the login screen to log into.
Your E16 will be accessible as always and wont be affected.... So your workflow shouldn't suffer.
As to the version E22 -> E23: Apparently the version is kept to the latest version available in the repo's, so could [will] change to E24 too. 
- A point to keep in mind:
When using E23, the login screen will become "darker" (i.e the backlight is dimmed) and that can be stopped in the E23 "settings panel", "look", "compositor" and checking "Don't fade backlight".
-
Another point:
Don't expect tricks and stuff from e16 to "simply" work too in E23. Use the E23 settings panel as much as possible to alter or modify things ... otherwise it might get messy.
-
Happy testing.
