In the Megila twice the name Charvona is mentioned. However there is a difference - Once it is spelled with an Aleph at the end and once it is spelled with an Hai at the end? Were there two Charvona's or was it the same person? If it was the same person, then why the spelling change?
It was the same person.
Many times we find people in Tanach mentioned in different places with their names spelled differently. Like Chizkiyahu sometimes has a yud in front of his name making it Yechizkiyahu, or Tzidkiyahu is sometimes Tzidkiyah without a vav at the end. In Megillas Esther, we find Bigsan becomes Bigsana later, or as you pointed out, Charvona with an aleph becomes Charvona with a hay. Although these differences do not mean we are referring to different people, they may yet have deeper meaning attached to them.
With regard to Charvona specifically, Maharal writes that it is spelled with a hay later in the megillah to hint that he was influenced by God to speak up to Achashverosh concerning Haman's misdeed. (Letter hay is symbolic of God.)
According to Medrash Rabba Esther 10:9 the second time it says Charvona it is written with a Hai as it was Eliyahu Hanovi in the guise of Charvona.
As Gershon answered, the Midrash identifies the latter Charvona spelt with a Hey as Eliyahu Hanavi. (The Rama (OC 690:4) cites this as the basis for the phrase "וגם חרבונה זכור לטוב" - "Charvona who is remembered for good" in Shoshanas Yaakov, [although the Gemora says that the original Charvona was in fact evil and part of Haman's wicked plot]; since this was really Eliyahu Zachor Latov disguised as Charvona).
The Alshich (Masaas Moshe to Esther 7:9) questions how the Midrash could possibly claim that Charvonah was Eliyahu, as the verse explicitly states that he was "one of the chamberlains before the king". He explains that it is actually derived from this very phrase. The words "before the king" seem superfluous, as it would have been sufficient to just write that Charvona, one of the chamerlains said. Rather, expounds the Midrash, ony "before the king", he apeared to be "one of the chamberlains", but in truth he was Eliyahu Hanavi. He writes that this is also alluded to by spelling Charvona differently from his prior mention in the Megilla, to hint that it was not the same person. He adds that the letter Hey corresponds to the five letters in the name אליהו.
The Shu"t Mayim Chaim (Siman 22) explains why specifically the letter Aleph was changed to Hey: There is a law in Hilchos Gittin (EH 129:34) that names in Loshen Hakodesh are spelled with a Hey at the end, whereas names in foreign languages are end in an Aleph. Therefore the wicked chaimberlain was named חרבונא, while Eliyahu Hanavi was called חרבונה.
The Chida (Kisey Rachamim, Mesechtes Sofrim 14:6) brings from the Arizal that Charvona is equivalent in Gematria to "פינחס אליהו". (See Kaf HaChaim (OC 690:100) who explains that this Gematria works out by adding the ten letters of "פינחס אליהו" and the כולל).
See also the last point to my answer here.