I currently have a unlimited subscription for another 42 days. I received this subscription also with a gift card. Now I bought a new gift card for myself (happy me) and I want to upgrade to Premium, but when I fill in the code and choose 3 months premium upgrade it get the message 'We are currently not able to redeem this code'. Does anybody has this problem?

I probaby has something to do with Payment Method. I don't wanna use Paypal or Visa.

It's also saying:

Please select which Spotify product you would like to redeem below. If you choose to upgrade to Spotify Premium your remaining 41 days of Spotify Unlimited will be converted to days of Spotify Premium.

So it probably is possible.