Hi @ahmmu18, 

welcome again to our English Community. 

Thanks for your message. 

Yes, this is absolutely possible. You just need to port your prepaid-number to o2 postpaid. 

Please follow the instructions that you find here: 

I hope these instructions are helpful to you. 

Loving greetings,

Tobias

The instruction is for moving the phone number from another provider to o2 prepaid and not from o2 prepaid to o2 postpaid!

@ahmmu18 : First order the tariff you like: www.o2.de

After you have received your new SIM card, you have to activate it and you will receive a new mobile phone number. You use this to register at Mein o2. You should already know the process from your prepaid card, right?
Then go to the section "Number portability" and enter the data you have given for your prepaid card (name, date of birth). Bisheriger Anbieter enter “o2-de” and select “sofort”
Then you wait for the MNP.
Porting is free, your prepaid card wil switched off after porteing, your existing credit is transferred from the prepaid card to your new contract. 

Thank you @o2_Tobias for the detailed answer.

I tried to do that, but got an error message that says:

Die Mitnahme der Rufnummer von O2DE zu O2 ist leider nicht möglich.

Wenn Sie mit Ihrer Rufnummer neue Konditionen nutzen wollen, können Sie mit Ihrem bestehenden Vertrag eine Vertragsverlängerung durchführen.

Not sure if there’s anything I can do about it.

Have you already signed a postpaid contract and have already received the SIM card?

Thank you @o2_Tobias  and @tearsbeforecrying for your help.

I’ve went to an O2 shop (at Friedrichstraße Sbahn) and they guys there did amazing job. They helped me doing everything I needed to in no time.

All I have to do now is wait a few days and things should work just fine.

Hello @ahmmu18 and thanlk you for your feedback :-)

If I understand you right our colleagues in the shop were already able to help you and your phone number will be transfered to your postpaid contract soon? That is nice to hear :-)

Regards,

Lars