Hi, Raveen. I think (but can't guarantee) that Bulk head and exit seats are held back for allocation at the airport. Upgrades are a different matter, which has been discussed extensively on v-flyer since OLCI started. You'll find some of the threads that cover it using the search tool (which is your friend:D ).
I have been upgraded after using OLCI, so I can guarantee that it does happen but, as you will see mentioned often in these threads and in the
upgrade guide here you'll see that the best aid to an op-up (free) upgrade is status in the form of a gold or (at least) a silver FC Card.
Best wishes
David