I realised I needed more data, so I upgraded to a more expensive plan
I'm now without Internet for 2 days because my new plan "will renew" in 2 days. However I'm pretty sure I'll be paying for it from the day I ordered it.
I'm equally sure that if a new plan on a digital sim or an existing ID sim, is ordered from scratch, it starts instantly.
I feel that the delay is much more about taking advantage of knowing that someone has run out of data and would therefore need to buy a vastly more expensive "add on" to cover their waiting time, rather than just start their new plan immediately. I don't believe the delay is a technical necessity.