Query Regarding Capital Gain

I am in CA Inter and started the chapter of Capital Gain.

Conversion of debenture into share is not treated as Capital Gain u/s 47.  Also for debentures and bond the benefit of indexation is not available.

Now, suppose X purchases convertible debentures in Aug 2002 and it get converted into Equity shares in Sept 2012 and then he sells those equity shares in 20-21. So, how we will do indexing? Whether we will take index of 02-03 or 12-13?