While traveling from A to C, if you chose your boarding point as B, then you lose the right of traveling from A to B.
If in the near future, you need to travel from A, instead of B then depending upon the category of the train, you need to purchase a second class unreserved or 1st class unreserved ticket. Then check the chart if any passenger has been allotted your berth for A-B. Also search for any vacant berths which is vacant for A-B. Then go to the TTE and request him to allot the said berth (If found vacant from the chart) else any other empty berths for A-B. Do tell him that your situation which made you board from A instead of B. If berths are available, he would allot you the berth and charge difference of fare for the journey leg A-B. Else, if unreserved coaches are available in the train, then he would ask you to move to the unreserved compartment (If vacant berths are not available).
Do take special precaution for trains like Rajdhani/Shatabdi/Jan Shatabdi where unreserved compartment are not available.
You may also try your luck at the Current Reservation Counter, if there exists any at Station A to purchase confirmed ticket at the last moment, subject to availability.