1&1 DSL tariffs change when again possible?

I have recently extended my dsl contract at 1&1 and selected as a premium a Nintendo Switch. My contract is now running again for at least 24 months. My question is when can I switch back to a new tariff? I would like to have an iPad as a bonus bonus. Could I call there and they would do that?

With us it was like that, we should cancel before (about 4 months); 1&1 then called us a few weeks before and then we have a new contract with takeover of the old phone number completed,

He was even a little cheaper and as a "condition" we received the tablet.