How to make separate projects for existing .cs files

I have a solution with one project in it.  That project has about 40 .cs files.  I want to take those 40 files and group them as to function, then make each group a separate project which I will then build into a .dll file.  I want to do this for ease of maintenance - when one of the projects code changes, I just have to rebuild it and upload just the .dll to replace the previous one in the installer program, thus making an update cleaner and smaller to download.

I believe this will work, but looking to the experts for advice.