I'm assuming you mean "use Burp's built-in functionality, such as Change request method", when you say "editing HTTP... with burp" (as opposed to "manually"), and that just changing the request text in the edit box is the "manually" you mean. If that's wrong, please clarify.
Burp does a number of things quietly and automatically, which you might not realize....
Finally I got it working.
The Problem :
The thick client wasn't respecting the mobile's proxy setting and 'invisible proxying' didn't help either as the app was using direct IP to communicate. ( This part took me a lot of time figuring out, rest part was almost ~20 minutes )
How I got it working :
Since I had a rooted phone I used ProxyDroid ( with ...