In Mathematica, licensing information is usually stored in a file called 'mathpass'. There are three typical locations where this file can live: In the installation directory under Configuration/Licensing, in a system-wide directory called $BaseDirectory and in a user-specific directory called $UserBaseDirectory. The platform specific locations can be found in the documentation, I include the links at the end of this post.
To change to a different license manager server you will need to edit this mathpass file directly, for example with a simple text editor.
Assuming you can can still start and run Mathematica you can also find the mathpass file by evaluating this:
FindFile[$PasswordFile]
And look at the content of the file with this:
FilePrint[$PasswordFile]
Or open the file with an external editor with this:
SystemOpen[$PasswordFile]
http://reference.wolfram.com/mathematica/ref/$InstallationDirectory.htmlhttp://reference.wolfram.com/mathematica/ref/$BaseDirectory.htmlhttp://reference.wolfram.com/mathematica/ref/$UserBaseDirectory.html
