I mess around quite a bit in building shared parameters to make our "database" more robust and have found a couple of things that can help those of us that break the rules and manually edit shared parameter files.
I'm a poor typist and often have to go back in and correct spelling. So I'll have to remove an old shared parameter and replace it. Rather than just removing it in total - you can comment it out. Just put in a # at the start of the parameters line (#PARM...) and Revit will ignore that line. This helps if you do need to use the old parameter for older projects. You could go back in and uncomment the line.
Also Revit seems quite tollerant of changing the parameter name associated with any GUID for NEW projects or templates. However, if you load a parameter in a project, then later delete the parameter from the project. Change the name of the parameter in the shared parameter file and try to reload it - Revit crashes. It seems that Revit projects hangs onto the GUID/parameter even though it was removed from the shared parameter list. It would be nice if Revit released the GUID so you could reload the parameter without rebuilding it from scratch.