we should use kv if we can. I agree the previous was wrong, but how about

  // preserve kv if we can. We know that kv matches key and value
  // and we know if kv ne null that the value is eq, so if the key is also eq 
  // then we can retain the tuple
 else new HashMap1(this.key, hash, value, if (this.key.asInstanceOf[AnyRef] eq key.asInstanceOf[AnyRef]) kv else null)`

Agreed, it ought to be if (this.key eq key) kv else null.

This change look wrong to me, and the original looked correct
This is the case where the hashes don't match (as I see it) so kv._1 is unrelated to this.key

As I see it this would cause a corruption in the tree, as the new value would be lost
so - conceptually

val previous = HashMap1(hash1, "a", "A")
val result = previous.updated("b", "B")
//which calls previous.updated(hash2, "b", "B", null, null)

this would give us a
HashTrieMap containing
previous and HashMap1(hash2, "a", "B")
and should be
previous and HashMap1(hash2, "b", "B")

I guess there isn't a test for that, or I am misreading the code

The change that you mane is better that the original ...but

It looks to me like the new and old code don't cope with a different value in the map for the same key, and convert to a hashMapBuilder when we could still use a Map4.
Also we could remove the creation of a Some via use of a getOrElse

something like

in object Map

private val flag = new AbstractFunction0[Any, Any] {
  def apply() = flag
}

and here

val valueOrFlag = elems.getOrElse(elem._1, Map.flag).asInstanceOf[AnyRef]
if (valueOrFlag eq Map.flag) {
  //its a new key
 convertToHashMapBuilder()
 hashMapBuilder += elem
} else if (valueOrFlag ne elem._2.asInstanceOf[AnyRef]) 
  // its an existing key with a new value 
  elems = elems + elem
//else as key is == and value is eq, its no change

👍
new tests - Looks like I have some reading and understanding to do!