string is an alias in C# for System.String.
So technically, there is no difference. It's like int vs. System.Int32.
As far as guidelines, it's generally recommended to use string any time you're referring to an object.
e.g.
string place = "world";
Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.
e.g.
string greet = String.Format("Hello {0}!", place);
This is the style that Microsoft tends to use in their examples.
It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.
AssemblyVersion
Where other assemblies that reference your assembly will look. If this number changes, other assemblies have to update their references to your assembly! Only update this version, if it breaks backward compatibility. The AssemblyVersion is required.
I use the format: major.minor. This would result in:
[assembly: AssemblyVersion("1.0")]
If you're following SemVer strictly then this means you only update when the major changes, so 1.0, 2.0, 3.0, etc.
AssemblyFileVersion
Used for deployment. You can increase this number for every deployment. It is used by setup programs. Use it to mark assemblies that have the same AssemblyVersion, but are generated from different builds.
In Windows, it can be viewed in the file properties.
The AssemblyFileVersion is optional. If not given, the AssemblyVersion is used.
I use the format: major.minor.patch.build, where I follow SemVer for the first three parts and use the buildnumber of the buildserver for the last part (0 for local build).
This would result in:
[assembly: AssemblyFileVersion("1.3.2.254")]
Be aware that System.Version names these parts as major.minor.build.revision!
AssemblyInformationalVersion
The Product version of the assembly. This is the version you would use when talking to customers or for display on your website. This version can be a string, like '1.0 Release Candidate'.
The AssemblyInformationalVersion is optional. If not given, the AssemblyFileVersion is used.
I use the format: major.minor[.patch] [revision as string]. This would result in:
[assembly: AssemblyInformationalVersion("1.0 RC1")]
Best Answer
Use Reflector and its plugin Reflexil; of course, if the assemblies are signed they won't work together anymore (actually I think they won't work at all?). Otherwise you won't even need to modify the references I think.
Edit: also, Marc is absolutely right; can't imagine what scenario would absolutely require you to do this, it's quite hackish.