IIRC, Hawkeye changed to add more firepower to the Avengers line up... then changed back when he was no longer needed. Carol Danvers was Binary for a while.... I think she changed when she beat her alcoholism during the Busiek era, but it might have been slightly before. She later became Captain Marvel as a 'promotion'.

Monica Rambeau changed like a million times, mostly for corporate marketing and not really any in story reason.

You're correct: Clint was Hawkeye, then Giant-Man, then Hawkeye again, then Ronin, then Hawkeye once more. The switch to the Ronin identity occurred after Clint was killed in Avengers disassembled and brought back to life; not only was he suffering from psychological issues, but someone else (Kate Bishop) had meanwhile adopted the Hawkeye moniker in his honour.

Hank Pym naturally went through many identities: Ant-Man, then Giant-man (due to his new ability to grow to giant size, natch), then Yellowjacket (when he suffered a nervous breakdown), then simply Dr. Pym, scientific adventurer (when he tried to give up costumed identities, which he viewed as a factor in his ongoing psychological problems), then Giant-Man again, then the Wasp (in honour of his supposedly-deceased ex-wife), and nowadays I have no idea who he might be. I think he's become Ultron, in fact, but someone would have to confirm it!

Captain America gave up the name when he had problems with the country's politicians (during Watergate, or more exactly during the first Secret Empire storyline) to become Nomad, the man without a country. After a stint as Cap again, he once again ran into trouble with the government and simply became "the Captain", with a red a black variation of his old costume. In recent years, after being killed and resurrected (there's a trend, here!) he gave the Captain America identity to Bucky and became Steve Rogers, the super-soldier (and head of whatever had replaced SHIELD and HAMMER at the time). He's back to being Cap now, unless he's dead again.

Many of these transformations make sense, but few are those that hold for good. It's more common for another character to pick up the mantle of an established hero than for one to change their heroic identity for the duration.

Speaking of Teen Titans, Mal Duncan had a bit of an identity crisis. He was introduced as Mal Duncan, a streetwise kid. For a while, he used on exo-skeleton as the Guardian, based on the old Simon & Kirby hero. He received a mysterious horn and became Hornblower. The names never really stuck and he was mostly known as Mal, before being written out. He was retrofitted back into the Titans, post-Crisis, with a slightly altered history, with an electronic horn, instead of a mystical one.

And Binary.

You just had to ruin that for me, didn't you.

He also called himself Goliath for a while. I forget why; was it in reponse to the time when his growing powers as Giant-man were limited?

Captain America has changed identity a couple times, for good story reasons - first he quit being Cap because he was disillusioned with the country after Watergate in the 70's, and became Nomad, the man without a country. Then in the late 80's, he resigned as Cap after the Reagan administration tried to force him to become a government operative under their control; on that occasion, he continued fighting evil as The Captain, while the government got someone else to play the role of Captain America.

The Cat became Tigra after she was turned into a literal cat-woman by a race of cat demons from another dimension. Really.