Amending Chart of Accounts

Dear All

At the moment we have entity code(Segment 1 i.e. balancing segment) as 3 characters. Due to some business reasons my client want to increase the length from 3 to 5 characters. I did some research on that.. there are some solutions like creating secondary ledger and then perform the mapping from old to new COA but in this process I guess we will lost the drill down functions. Can  any one of you please suggest what may be the most efficient way of achieving this amendment ? we are on R12.1.2.

Thanks

Pawan