I am in the process of sorting out share info for my CSR shares dating back to 1997. When CSR and Rinker demerged in 2003, I find this on the ATO website:

How do I calculate the cost base and reduced cost base of my post-CGT CSR and post-CGT Rinker shares?
Regardless of whether or not you choose rollover, you must recalculate the cost base and reduced cost base of each of your post-CGT CSR shares and each of the post-CGT Rinker shares you received for those shares.

The cost base of these Rinker shares is not $1.53.

I understand all but "choose rollover". Can someone please explain it to me? I have not sold the shares, but I do need to keep the cost base accurately.

Thank you.