There is a connection between between "elf" and "changeling".
It relates the reallife concept of "fairy".
There are at least two concepts of changeling.
One is more obscure, where the "changeling" is the offspring of a human and an elf during a (shamanic) magical encounter.
The other changeling is a widespread belief of the fairy folk stealing a human baby and leaving a fairy baby in its place. In this sense, the changeling refers to the exchange of babies. It is a fairy who grows up among humans.
The reasons for stealing human children vary, some innocent some sinister.
Shakespeare mentions how the more powerful a fairy is, the younger (and thus less morally and ethically mature) the fairy appears. So the king and queen are fairy spirits in the form of toddlers! The queen stole a human child to play with, thus also granting the human toddler immortality.
In British folkbelief, the fairy was any "magical" spirit, but especially a spirit of magic itself, relating to fate. From southern English influence, a fairy came to be identified as a specific kind of magical creature, the one that Shakespeare describes. It is a normal humansize spirit, often a child or teen, but as a spirit can shrink to some degree like angels can to dance on the head of a pin. The most powerful fairies can shrink to the size of a fingernail. Eventually, these fairies came to be visualized as normally tiny.
Thus in contrast, the "elf" specifies the earlier concept of the kind of fairy that is normal humansize. The elf is a kind of fairy.
In D&D 1e, this humansize "faerie" is identical with the grey elf. In later editions of D&D, this faerie is identical with the "sun elf" and the "eladrin" elf.
By extension, the 5e fey changeling can be a fey eladrin. Or at least, there is some kind of relationship.