If you switch to a subscription, you'l lose your calling credit. If you have an automatic payment set up, it's best to cancel it.
You can do this via your account on our website if you go to My Viking > settings > payment settings. Then, you can use your current bundle and your remaining calling credit (at € 0,10/MB and € 0,10/SMS)
Afterwards, you can switch to a subscription via your account: go to My Viking > settings > SIM card settings > Switch to a subscription.