cancellation of DSL connection

To cancel your o2 DSL contract at the agreed end of the contract, please complete the cancellation form, sign it and send it to o2 by letter or fax.

I can only advise against this approach. A tariff change from a fixed-term contract to a flex contract is not offered by o2. 
When ordering a new o2 DSL connection, you will therefore receive a new telephone number and the connection price of 69.99 EUR will be charged. Since you are not a new customer, you would not receive any new customer discounts.
Routers from o2 must be returned to o2 at the end of the contract. If you want to rent a router again together with the new Flex contract, you will be charged 49.99 EUR provision fee, 9.99 EUR shipping costs and the monthly rental fee for the device you want.
If you cancel your existing connection and then order a new connection, you have to expect an interruption in the supply of Internet and telephone services that may last for a longer period of time. 

I would recommend to change the tariff to o2 my Home L after you have moved to your new place. This way you can see what speed is actually possible at the new address. The price should be similar, as there is only one discount: Either a discount on the moving fee or a discount on the new tariff.