Voucher numbering

There is no such rule. But it is part of Internal Control that all vouchers are prenumbererd and serially numbered. While conducting Audit, If auditor notices that you can modify voucher numbers later then he would deduce that internal controls are weak. There is no legal violation per say. But I suggest you pass such entries with some different series. say BL/2015/1/1, BL/2015/1/2, BL/2015/1/3 if these are vouchers then you can add one marginally changing the series say BL/2015/1/1, BL/2015/1/1A,  BL/2015/1/2, BL/2015/1/3