I have booked a ticket on a train from Mumbai to Chennai and boarding at Pune. Since my schedules have changed, I will be in Mumbai on that day. Can I board the same train in Mumbai in my own ticket and claim the seat/berth?
No, you can't. Simply because the train is from Mumbai and you're boarding at Pune doesn't mean your seat will be empty on the Mumbai to Pune leg.
Changes of boarding station are only allowed if the station you want to get on at falls later than the one mentioned on your ticket.
CHANGE OF BOARDING POINT
If a passenger desires to occupy reserved accommodation from any station enroute, he would be permitted to entrain at any intermediate station he chooses irrespective of the distance from the starting station subject to the following conditions:
- A specific request must be made in writing at the station from where the ticket is purchased and reservation made at least 24 hours before the scheduled departure of the train from the starting station.
- The Railway Administration reserves the right to use such accommodation from the originating station up to the station at which the passenger is due to entrain.
- No refund will be permissible for the portion of journey not performed by the passenger.
Unfortunately, in this case it means you need to book a new ticket. If you already have a confirmed Pune - Chennai leg, I'd recommend just buying a Mumbai - Pune leg.