Dear Sweta Jalan you've raised a very good & interesting query. The solutions are:
1) You need to file a correction / revised return for the wrong entry of PAN. I totally disagree with Abhinav Agarwal sir's comment to ignore the 'inconsistency letter' from NSDL. If you don't make the correction then the person whose PAN has wrongly been conveyed to the I.T. dept in the TDS return won't get his credit for the TDS deducted from him/her.
2) There's no time limit for filing of Correction / Revised returns, but you've to check that whether the challan has been booked under the OLTAS of TIN-NSDL or not. If the challan has been booked then you've to contact your A.O. for the correction. (For details log on to www.tin-nsdl.com)
3) Dear Manjunath There's no need to change the Acknowledgement number in Form 16, the NSDL people can track the details with the help of actual / regular Provisional Receipt Number.