Is it possible to downgrade my o2 DSL contract?

Hello,

I have a o2 DSL All-in L contract and would like to downgrade to o2 DSL All-in S.
I want to know how much it costs to perform the downgrade and if I do the downgrade I can then terminate the contract when I want (since it is Ohne Mindestvertragslaufzeit).

Thanks in advance.

Kind regards,
Rui