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.
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.