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

Yes. You can do this via product settings on the website, or in the My Viking app.

Will you switch immediately? In that case, your subscription bundles will remain valid until their expiry date. You'll use those first, and only when they're used up or expired you'll start using the bundles of your top-up. You can also choose to switch as soon as your current subscription bundles expire. 

Please note:

You cannot switch if you still have unpaid invoices/payment invitations. Consult your balance via My Viking. Due to the obligatory registration of prepaid cards, we ask you to immediately top up via bancontact. You can switch up to 6 times between a postpaid subscription and a prepaid SIM (or the other way around) per year.