Do I have to remove the license on zLib-licensed code project in order for it to be integrated into a GPL3, Apache2 or MIT-licensed codebase?

Question

As the title states, if I take one code project that is under a zlib license, and incorporate it into a GPL3, Apache2 or MIT-licensed codebase, do I have to remove the license from the code project prior to incorporation?

If so, do I also have to change the license of the original code project, in order for the licensing to not conflict with the larger GPL3/Apache2/MIT codebase (since they now share the same code)? Or can I effectively 'grant' use of the code (if I'm the sole owner) under those licenses, without changing the license of the original code?

I realise I am in essense asking 3 different questions here due to the differing conditions of the 3 licenses, but I appreciate any general advice given.

zLib is a more permissive license than the others, and is stated to be compatible with the GPL3.

Answer 1

As the title states, if I take one code project that is under a zlib license, and incorporate it into a GPL3, Apache2 or MIT-licensed codebase, do I have to remove the license from the code project prior to incorporation?

No.

The ZLib license gives anyone the right to include the code into a differently licensed project, but only the copyright holders can (jointly) decide to remove the ZLib license from the code.

If the code is already well-known to be available under the ZLib license, then even if you are the sole copyright holder who incorporates it into a differently licensed project, I would recommend that you do it in the same way as a non-copyright holder would do it. The main reason for this is that is completely avoids questions if you had the right to drop the ZLib license.

Or can I effectively 'grant' use of the code (if I'm the sole owner) under those licenses, without changing the license of the original code?

As sole copyright holder, you always have the freedom to offer your code under additional licenses (even licenses that are incompatible with each other) and you can make that decision at any time.