Diary of a Vampeen by Christin Lovell. It is the first book of her Vamp Chronicles series that currently runs to seven books and two novellas.
Imagine living a human charade for fifteen years and never knowing it. Imagine being provided less than a week to learn and accept your family’s true heritage before it overtook you. Alexa Jackson, Lexi, is abruptly thrown onto this roller coaster and quickly learns that she can’t change fate, regardless of how many lifetimes she is given. She will be transformed into a vampeen on her sixteenth birthday, she will be called upon to fulfill a greater destiny within the dangerous world of vampires, and she will have to risk heartbreak and rejection if she ever wants a chance at love with Kellan, whether she likes it or not.
Lexi's best friend is indeed Mel. I don't think we ever learn her surname - skimming the first couple of chapters she is always just referred to as "Mel". The only mention I can find of the new wardrobe is when Kellan is describing the transformation to Lexi:
“You sleep through everything. It’s like when you have surgery, the anesthesia makes you sleep deep enough to ignore the pain,” he explained. “After, when you wake up, it is shocking at first. Everything is intensified. You hear every pin drop and smell every daisy three yards down, but you adjust after a few days. You’ll feel weird. It’s almost like an out-of-body experience in the beginning. You’ll have to get to know your new figure and lifestyle. You’ll have to buy a new wardrobe.
I cannot find any mention of Kai or Auggy, but I only have the first book and they may be in later books.