Can I switch from a prepaid SIM card to a subscription?

Yes. You can do it via My Viking. 

Are you switching immediately? In that case, your bundle will remain valid until their expiry date. You do lose your calling credit when you switch. You can also opt to switch when your calling credit reaches € 2.50 or less. In that case, make sure to also switch off your automatic top-up if you have one, also via My Viking. If your bundles are still valid when you switch over, they will remain valid until their expiry date. 

You can switch up to 6 times between a postpaid subscription and a prepaid SIM (or the other way around) per year.