Some time ago I developed a small project for use in unit testing scenario's and published it on Codeplex. Back then I choose the GPL license but I didn't really understand it. Today I changed it to MIT, because it's a much simpler license that I can understand.
I like to be recognized as the copyright holder, but I want everybody to be able to use the library without worrying about license issues. I think MIT covers both, but not 100% sure. 'Virality' is not an issue for this project.
Would this license (MIT) in any way prevent a person to use my library for the unit testing of a closed-source commercial or private project? In this regard, is my recent change to MIT a change for the better?
The MIT license is essentially a "do whatever you want with this software, just don't blame me if something goes wrong" license. It's quite liberal. So if you don't care how your library is being used (or abused) and just want to be recognized as its author, then go with the MIT license.
The answer to your question is right there in your license:
Permission is hereby granted, ... to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.
So no, you have pretty much assigned the right for anyone to do as they please with your code, as long as they attribute you.
If i understand what happens regarding two licenses that you defined at two different points? the answer is:
The simplest approach will be:
You should keep the original license of earlier release. For the next release you can release with MIT license.
Alternatively, you can re-release the same software with what is called Dual license. You can refer to this discussion which explains some good parts of it: MIT vs. BSD vs. Dual License
There is another thing i want you to clarify: Why do you want to change from GPL from MIT? Only advantage i see is that MIT will allow redistribution under commercial easily. But i would rather want that if people use my work - their derived work may well be open. If in case, there is a friend of mine whom i want to give for commercial purpose - i can always make specific distribution under MIT (because i am the copyright holder).
So my general advise to you is :
Keep GPL license for the mass who can directly download. If you need to give special rights to people - replace the license file and distribute to them personally.