You will first need to upgrade your Studio One license via PreSonus’ my.presonus website before inputting your serial into Studio One.
Log into My PreSonus
Select My Products > Software
Select “View more details” for Studio One Professional OEM
Click on “Upgrade” button
Paste or enter the new product key and click “Upgrade”
Your product key will be shown here: splice.com/plugins/your-plugins
You can then enter the new serial into Studio One directly.