Jeremiah is ירמיהו and means "Whom the LORD has appointed." It appears even the LORD uses this name:
And the word of the LORD came to me, saying, “Jeremiah, what do you see?” And I said, “I see an almond branch.” (Jeremiah 1:11) [ESV]
ויהי דבר־יהוה אלי לאמר מה־אתה ראה ירמיהו ואמר מקל שקד אני ראה
This is how his name is spelled until Chapter 27:
In the beginning of the reign of Zedekiah the son of Josiah, king of Judah, this word came to Jeremiah from the LORD. (27:1)
בראשית ממלכת יהויקם בן־יאושיהו מלך יהודה היה הדבר הזה אל־ירמיה מאת יהוה לאמר
The alternate spelling continues to be used (28:5, 6, 10, 11, 12, 15; 29:1) until 29:27 when the original spelling resumes and is continued to the end. Note: this question asks which of the two is correct: What is the Hebrew name of the prophet Jeremiah—Yirmeyah or Yirmeyahu?
What is the reason and/or significance of changed spelling? The original spelling resumes after Jeremiah's letter to those who were in the first group of exiles and the alternate spelling is also used by Daniel (9:2). Does location (i.e. Babylon) contribute or have anything to do with the spelling? How might the meaning of the name change by dropping the vav?