Take O2 number forward from old O2 contract to new O2 contract

Your plan is possible with this form: internal porting form

Once the new contract is active, you request number porting at the end of the old contract. Your number will then be transferred to the new contract at the end of the old one. Cancel the old contract beforehand.

https://info.o2online.de/kuendigung/

Thanks for the reply. 

Can you clarify the step by step process? 

Lets say to terminate the current contract (minimum term until end of Feb 2026) i need to submit termination request by end of Jan 2026 with a one month notice so that the current contract can be terminated by end of Feb 2026. 

In this case I can only request internal posting request before the end of Feb after which my SIM card will become inactive. 

So does it mean that I will be apply and purchase a new contract immediately so that I can request a porting request? Which means I woll be having dou le contract st the same time and paying more money for two contracts. 

Can you clarify me on the steps? 

Correct, you should submit your cancellation as soon as possible and sign the new contract in the meantime. You'll then pay for both plans for approximately six weeks. You could sign the new contract with your desired start date in mid-February. Then you'll only pay double for two weeks.