As simple as I can make it -
3.60 - You can use Henkaku and Enso
3.65 - You can use h-encore and Enso
3.68 - You can use h-encore
Henkaku and h-encore are basically the same thing, just installed in different ways. Henkaku is installed using a browser exploit, which is very easy to do. For h-encore, you need to get the app on the Vita using QCMA, which is not exactly difficult it's just a lot more steps and requires logging in with an active PSN account.
And Enso is the exploit that runs at boot to automatically load Henkaku, so you can have autoboot on 3.60 and 3.65 but not 3.68.
Most people who are downgrading are going to 3.60 so that they have the easier web browser exploit, but if you have previously installed h-encore on 3.68 then you should already have the h-encore app on your memory card so you could just go back to 3.65 and just install Henkaku again, then Enso, and have autoboot CFW with the advantages of the higher version of system software (so any 'stability improvements', native compatibility with any games that require 3.65, plus the latest version of VitaCheat will work).