IANAL either but I believe that if you release code under license A you can relicense it to license B, but anyone that obtained the code under license A will be able to keep on using it under that license.
A license is like a contract, and you can’t change a contract out of the blue without the permission of both sides. Otherwise licenses would be useless, as at any point someone could go “aha, I changed my license to the give-me-all-your-money license and you’re using my code, so pay up!”.
Since the license A in this case is an open source license, anyone that has a copy of the software with license A (such as amazon) will be able to share it again under license A. So effectively the old code base will always remain under license A, and only new changes will be fully protected by license B.
A license is like a contract, and you can’t change a contract out of the blue without the permission of both sides. Otherwise licenses would be useless, as at any point someone could go “aha, I changed my license to the give-me-all-your-money license and you’re using my code, so pay up!”.
Since the license A in this case is an open source license, anyone that has a copy of the software with license A (such as amazon) will be able to share it again under license A. So effectively the old code base will always remain under license A, and only new changes will be fully protected by license B.