Map transformation with dynamic keys

a!localVariables(
local!map1: {
a!map(Id: "Map 1", Key1: "A", Key2: "C", Key3: "C"),
a!map(Id: "Map 2", Key1: "C", Key2: "A"),
a!map(Id: "Map 3", Key1: "C", Key2: "B", Key3: "C")
},
local!map2: a!map(Key1: "ky1", Key2: "ky2", Key3: "ky3"),
a!forEach(
items: local!map1,
expression: a!localVariables(
local!map1item: fv!item,
a!update(
a!map(),
a!forEach(
items: a!keys(fv!item),
expression: if(
contains(a!keys(local!map2), fv!item),
index(local!map2, fv!item, null()),
fv!item
)
),
a!forEach(
items: a!keys(fv!item),
expression: index(local!map1item, fv!item, null())
)
)
)
)
)/*
{
a!map(Id: "Map 1" , ky1: "A", ky2: "C" , ky3:"C"),
a!map(Id: "Map 2" , ky1: "C", ky2: "A"),
a!map(Id: "Map 3" , ky1: "C", ky2: "B" , ky3:"C"),
}
*/

A slightly different approach using the displayvalue function.

a!localVariables(
  local!map1: {
    a!map(Id: "Map 1", Key1: "A", Key2: "C", Key3: "C"),
    a!map(Id: "Map 2", Key1: "C", Key2: "A"),
    a!map(Id: "Map 3", Key1: "C", Key2: "B", Key3: "C")
  },
  local!map2: a!map(Key1: "ky1", Key2: "ky2", Key3: "ky3"),
  a!forEach(
    items: local!map1,
    expression: a!update(
      /* Populate an empty map */
      a!map(),
      
      /* Define the keys, trying to replace key names */
      a!forEach(
        items: a!keys(fv!item),
        expression: displayvalue(
          fv!item,
          a!keys(local!map2),
          index(local!map2, a!keys(local!map2)),
          fv!item
        )
      ),
      
      /* Picking all values from the list item */
      index(fv!item, a!keys(fv!item), null())
    )
  )
)

Thanks Stefan. I don't need to iterate over values part. my sleepy head did not catch that. 

Thanks Stefan. I don't need to iterate over values part. my sleepy head did not catch that.