I'm not sure I did it the right way, but here's how I handled the uncashed cheques when I transferred to a new system that was not very compatible with the old.

I didn't touch the opening balances.  I just calculated the total amount of outstanding cheques and created a journal entry for the same day as the opening balance:

Bank DR - Total Amount of Cheques

Bank CR - Created an entry for each outstanding cheque with the cheque number