You can copy a map element's expressions to another map element. To do this, select the map element, right-click and select Copy, then select the target map element and select Paste Expressions (Mappings). This copies all of the expressions, i.e. Loop expressions, Value expressions, etc. from the source map element to the target map element. If the expression is already present in the target element, it is not modified. Also, the expression copying applies to all children of the source and target elements that are matched by map element name (in exactly the same way mapping from an input map element with children to an output map element with children is done).
This is particularly useful if the output structure has changed under the map, by adding or removing levels of elements, or moving a block of elements. You can easily move the mappings by copying the expressions from the old locations of the elements to the new ones.