Base Unit Of Measurment in Material

Dear Experts,

Base UoM in material master is in Metric Tonne "MT",While Creating the BOM of the Material My Base qunatity is also in MT by default but I want to change Base Uom to in KiloLiter "KL".But system is not allowing to change.

In system there are many Transactions iof material in current period.

Please suggest as it is in production server.

Thanks,

GBS