Change the cardinallity ratio of a an already existing field from 1 to unlimited
It is possible to change an element with a singular cardinality via the YAML source without any data loss. You would definitely lose data going from multiple cardinality to singular.
In the YAML source, add the '#multiple': true property to the element.
name:
  '#type': textfield
  '#title': 'Name'
  '#multiple': true