Thanks for the reply. Here is what I found.
To port an existing app, you would just replace the Class_Name_With_Underscores to Class\Name\With\Underscores, so the port for users is not that big a deal.
Another thing one could do would be to write a PHP Script to mod the source code to do the port. This would render porting a much easier task for most people.
Yes, the change was/is a lot of work to do. I actually was able to automate most of it, but ran into issues with the unit testing. There is a lot of unit testing, so I think even a bigger issue is porting the testing to PHPUnit.
The reason to change to namespaces is simply to bring it up to modern standards. PHPOffice went through the same thing a while back, and is better for it. I think you have a good package, so wanted to help. My personal reason is that I use my own autoloader (fast as hell, zero memory footprint, and does not include unused classes like the include everything system requires), but it needs a logical namespace to file mapping, which most modern packages use. Composer will work with the package is it currently is obviously, but I don't use the Composer autoloader. Also, keeping the package modern is the only viable thing, as eventually it will become unusable due to PHP changing underneath it.
So at this point, I found another library to clean HTML (not as nice as yours), but it works for me.
I would be happy to submit my work so far, but I don't feel it is ready for release, primarily since it does not pass unit testing. I also had to make some minor namespace changes. Let me know if you want to see it.