use .NET Reflector, with the addin that allows you to write the disassembly out to a file. Unfortunately, you will lose the comments but you will get the latest source.
As a better, but more time-consuming alternative, there is another add-in that allows you to compare two assemblies, so you should be able to see what changes were made to the code by comparing them in NET Reflector and modifying the existing source accordingly to add in the alterations but leave the original comments etc intact.