If you have time, take a copy of e-ticket and id proof and change boarding point at nearest prs and ask for boarding point change.
However, generally for nearby halts, tte comes after crossing both the stations. confirm from co passengers if tte has come b/w tpj and srgm. you may go to tte immediately after boarding at srirangam, and claim your berth before allotment.
(this happened with me b/w mgs and bsb many times. tte had always came after crossing both the stations only.)