With the blog records provided by Hatti, we can understand that you can board a GR with II class ticket. In your case you already paid II class fare between A-B. You already changed your boarding point once per Hatti's information. So per logic you can travel between A->B in GR but will be allowed to board at A only if there are vacant berths....In which case, I am sure TTE will not ask you for an additional II class ticket or difference amount as you have already paid it in full. ...This is only my thought process and I might be wrong.... Only in this way it would be possible to utilize the total amount you paid......If this is not allowed per any existing rules,, then we can treat the money you paid in extra as just a favor to IR