HI Dilip Kumar Shaw,
If I am not wrong in the change object you are using ‘set’ option and updating the association( this is the reason why the data is getting over ridden), there will be two more options enable in your change object which is ‘Add’ and ‘Remove’( For Many- Many ) . You can use the Add and then set the association( this option will add the object in the association table on top of existing list) .