In a microflow use the change object to set the value of your attribute (second enumeration) like this:
if $object/enumeration1 = 'value 1' then
enumeration2 = "value 1" else if
$object/enumeration1 = 'value 2' then
enumeration2 = "value 2" else
Create a microflow with a custom retrieve on the enumeration you want to kill. Loop over the list and change these objects to set those to the other enumeration. Then afterwards you can delete that enumeration so you do not create more data problems.