transfer to o2

hello, I have 2 questions,

first , I  use freenet-funk but I want to transfer my number to o2 by buying  this  phone : “Samsung Galaxy S20+ mit Tablet” . But I did not see an option for that when ordering. How do I transfer my number,? before or after I ordered and received my phone and sim ? 

and second, if I choose 20 gb datenvolumen and then when i need more can i upgrade my tariff?  

waiting for your answer. thanks in advance!