Access Denied when migrating httpHandlers or httpModules configuration in IIS 7.5

While running the below command on one of my IIS 7.5:

%systemroot%\system32\inetsrv\APPCMD.EXE migrate config "Default Web Site/MyWebApp"

I was getting Access Denied error (80070005). As I had used that command in other IIS 7.5 servers to migrate the httpHandlers and httpModules configuration from Classic .Net to Integrated .Net mode, I knew the issue was environmental.
Turned out it was due the web.config being read only, as I used a source code management system that download as read only the files not checked out by the developers).
Removing the read only attribute from the web.config, fixed the issue.