I will fly with Norwegian from BUD to CPH in February. I have a LowFare ticket so the seat selection is €12. I know that if I don’t reserve a seat in advance, I will get one random upon check-in which is possible online 24h before departure. My question is: if I don’t want the randomly assigned seat after check-in, can I still buy another seat that time for a fee?
Yes, you will. All of this is on Norwegian's website, here. To quote:
"If you’ve purchased a LowFare ticket and you don’t really mind where you sit, then not to worry – we’ll automatically assign a seat for you on the day of departure. There will be a charge to change your seat if you’re unhappy with the one we’ve assigned to you when you arrive at the airport."
Note that the price will be a few dollars more: "LowFare: 12 - 26 USD (online) 30 USD (airport) per person, per leg"