Basically what I want to do it this: a pdb file contains a location of source files (e.g. C:\\dev\\proj1\\helloworld.cs). Is it possible to modify that pdb file
If you're looking to be more generic about the paths embedded in a pdb file, you could first use the MS-DOS subst command to map a particular folder to a drive letter.
subst N:
Then open your project relative to the N: drive and rebuild it. Your PDB files will reference the source files on N:. Now it doesn't matter where you place that particular set of source files, so long as you subsequently call the root directory "N:" like you did when you built it.
This practice is recommended by John Robbins in his excellent book, Debugging Applications for Microsoft .NET and Microsoft Windows.