Re-Enter CAS Mode to Edit Sim Later

1. Press Ctrl+Shift+C to bring up the cheat console.
2. Enter testingcheats true
3. Enter cas.fulleditmode (this enables you to make full edits, like when you start a new game)
4. Now click on a sim while holding Shift down. You should get an "Edit in CAS" option.
5. Click that option and you're ready to edit your sim.

Note that testingcheats true gives you access to all kinds of cheats on several objects when clicked with shift down (resetting sims, giving happy moods etc.)