Hey man, I just resolved the exact same issue. Thought I'd share my steps in case it can help you. Here's what I did:
1. Log into My.PreSonus.com, select "My Products". You should see your new Artist product there. If not, hit the little refresh button on the top right corner. This will allow you to see the product key of the Artist version.
2. Open the Studio One Prime application on your computer. Click on the "Studio One" in the top menu. Select "Studio One Activation".
3. Copy & paste your new activation code and make sure the radio button ("I have purchased an upgrade") is NOT selected. Then just walk through the steps.
Hopefully this works for you.
Cheers,
David