simply do this create form level variable
int FormLevelcmbbox1LastValue = 0;
now in combox's selection value change
cmbbox1_selectedValueChange(...)
{
ConvertFrom = FormLevelcmbbox1LastValue ;
ConvertTo = cmbbox1.selectedValue ;
FormLevelcmbbox1LastValue = cmbbox1.selectedValue ;
}
Happy Coding!
:)