Couple of years back they changed my H1b  to F1  easily. But now it may be difficult but depends on the case. I don't think it is impossible to change. If you do not have job in H1b from desi company they do not run the pay stub and at this time changing status from H1b to F1, USCIS may ask for pay stub. In that case without pay stub it may be impossible. In my case  they did't ask for pay stub. But for other people they ask. So, it depends on each case I think.