I've been using a Celcom postpaid 3G on a basic normal burner-type no-3G capability phone for years since I mostly use the internet on my PC and I have a tablet as a side. If I bought a smartphone and use that simcard on it, does it automatically charge me with a new plan? I'd hate it if I have to pay more for something I rarely use.

It was a First 1+5.

I thought about purchasing another prepaid number but my sister clarify to me that I just need to disable mobile data so I could use my postpaid number normally.