I'll try to give some background for why I chose to change the software license.
Subsonic has never been a typical open-source project. For all practical purposes I have been the sole contributor and committer. I always intended it to be a one-man project. From time to time people have kindly offered to contribute, but I have respectfully declined (because I prefer writing code rather than reviewing it). As a consequence I am in my full
right to change the license as I see fit.
Subsonic is clearly a commercial undertaking and has been for many years. I established a company (Subsonic AS) in 2010 with which I try to make a living. The source code is the only asset of this company, and it is problematic when the exact same code (minus the need for a paid license key) is made available by others. I've also registered that some forks of Subsonic are (or at least plan to) sell their own licenses. It has also been confusing for many users that they have to pay for an open-source product.
Then there is the issue I mentioned earlier about third-party licenses I have obtained and that are part of the source code. In the future I may use more commerical third-party components (e.g., html audio and video players). This is more complex to deal with if the Subsonic source is open.
As a service to people/projects who would like to use Subsonic as a basis to build their own stuff I made a github repo with the source of the last stable release (5.3) on https://github.com/sindremehus/subsonic. This contains 11 years worth of software development.
Regards, Sindre