Downgrading is as easy as installing a firmware. Suppose you are on 4.11 DEX. You can just grab the 3.55 Downgrade DEX from the ps3dev wiki and install it like any normal firmware and you get downgraded.someone should make a guide for this thing. >> including downgrading.
You have to be on 3.55 initially to do this hack though. After you've converted to DEX you can upgrade and downgrade freely.