Charles Ellicott Commentary Genesis 21:1

"And Jehovah visited Sarah as he had said, and Jehovah did unto Sarah as he had spoken." — Genesis 21:1 (ASV)

And the Lord (Jehovah) visited Sarah as he had said. —See Genesis 17:19, where it is Elohim who gives the promise. So here in Genesis 21:2 the name Elohim is interchanged with Jehovah.