I have applied for plan change from 1350UL to 1100ULF on 16/4/2010. But no change till today. Hope it will be today or tomorrow. But CC officers said it will be from next month. Actually which day, they don't knows.
Plan change need not take place in the 1st of every month. It mostly got sanctioned in the first week. Go to data.bsnl.in, postpaid orders, enter the telephone no in the prescribed format and post the screen shot. Make sure that you blacken the personal details before posting it.