Modified versions of the software are not the original version. The original version remains free.
I'm not sure this matters, especially not for this project, but it's not necessarily modifications. For example if you wrote an MIT licensed application it would be perfectly within my rights granted by the license to change nothing (or for example only the branding of the application), compile it, and distribute this for money without the source. Another issue with the MIT license which is once again very unlikely to be of relevance to this project is that it has insufficient patent protections. There are permissive licenses which do a better job protecting developers such as Apache 2.0 at discouraging patent trolls.