Did you use the advanced install? Changing the manager path should be coupled with an advanced installation and the instructions are here
http://rtfm.modx.com/revolution/2.x/getting-started/installation/advanced-installation
If you didn't, not sure exactly because the advanced install also allows for the core to be moved out of the install folder and the setup is significantly different than a normal install.
I wouldn't begin to pretend to know how to do this safely, sorry. I guess you can take a backup and try. See what happens.
I'd extract the files into their own folder on the server though, and then rename the folder or copy the manager files to your alternate manager folder.