When you upgrade FL Studio from Fruity to Producer Edition, for example. Or purchase new plugins. Then you must install a new license key. Open the HELP > About panel and re-enter your account login details, as shown here.
This is also true when you update FL Studio to a new version.
NOTE: It is not possible to give away or resell your original registration / serial number after an upgrade because you are still using it. We do not allow resale of accounts or products from accounts.
Image-Line Support Team