The Upcoming Vowel Merger in the Late Elizabethan Period

Before the 16 hundreds, words like nerve, bird, word, hurt and earth would have had different vowel sounds. The r in those words has completely altered the vowel sounds overtime. In these cases, the r sound was like a collapsed star at the centre of a black hole. And that's a major reason why english spellings are no longer fanetic. But when did that modern r start to replace the trilled r? Some scholars think that the upcoming vowel merger in the late elizabethan period was dependent on the modern r already being in place.