Prepaid Tarif change without changing number

Hello O2 Team,

My current Tarif is prepaid “O2 Loop Smart.”  I would like to change it to “O2 my Prepaid S.”

I try to change it on “Mein O2 App” on my mobile phone by clicking “Tarif wechseln,” but my desired Tarif does not appear (only “NEUER TARIF O2 Loop” is available to choose).

Instead, I can find this Tarif on “Angebote” page on App, and it seems that I can order it.

My questions are;

1. If I order new SIM card for new Tarif on Mein O2 App, from when (or at which point) would new Tarif apply?
2. Can I keep my current mobile phone number, after I change it to new Tarif?  If yes, how to process it and will it take some days to complete it?

Thank you in advance!