Zaruri ni hai.. Its completely on the basis of lottery system. I have got numerous time free upgrade. Mostly in Shram Shakti Express..SL upgrade to 3A and 3A to 2A.
Though their was waiting and no current availability still I got an upgrade. So kehna mushkil hai ki upgrade hi hoga. Better to Take a 3A ticket or wait for the current availability to reduce the fare of 3A ticket