I'm changing our webservices to an async model. And for that I have to change over a hundred methods.

Doing it manually is one (unappealing) option. Is there no way to programmatically parse & change multiple functions/code files?

Example:

[Webmethod]
public void MyWebservice (string parameter1, string parameter2, string parameter3)
{
  //Logic here
}

And change this to:

public void InternalMyWebservice (string parameter1, string parameter2, string parameter3, AsyncCallback callback)
{
  //Logic here
}

[Webmethod]
public void BeginMyWebservice (string parameter1, string parameter2, string parameter3, AsyncCallback callback, object asyncState)
{
  //Queue InternalMyWebservice in a threadpool
}

public void EndMyWebservice(IAsyncResult asyncResult)
{
  //Set return values
}

It's basically the same thing I have to do for each webservice. Change the name to "InternalX", add a parameter and create the begin & end method.