Hello Milind, thank you for updating me. I think you will have to try at the counter again. I don't think you can do it now because now IRCTC has taken over the reservation systems. So I cannot offer you any more now. Please try at the counter. I will also search and find out about MSTS, and try to give an answer. From what you say, even if you get a first class pass, if you want to use AC class, you will have to pay the difference, provided there is vacancy. That is the basic rule. Anyway I will check and post again. Meanwhile please also re-search (not research) and let us all know. Good wishes, and have a good day.