one point of view is if u r travelling with rac ticket for end to end, you hv good chances of getting the seat midway,
Example you have rac in LTT-ERS Duronto for LTT to ERS but some seat gets vacant at Madgaon or Mangalore, tte automatically should allot yout that seat is not booked for Mangalore to ERS. For, that it is important to keep in touch with tte. Mostly they dont charge extra but there are cases where in two rac passengers give half-half money and one of them goes away to tte, pays him extra and gets fully reserved new seat. That what I have seen in Nagpur Duronto Express.