Thanks for the input, all!! Looks like I’ve got lots of reading to do. happy dance
Rick Jones has been a Bucky stand-in (with mask), and has been linked with the Kree Captain Marvel, & been the Hulk’s (non-uniformed) sidekick.
Not sure if this is what you mean, but in one story Chris King dials H-O-R-R-O-R and turns into a monster. There may have been other such instances.
In addition to being Ant-Man and Giant-Man, Hank Pym also went buy the names Goliath and Yellowjacket. He also adventured under his own name for a while.
Not exactly what the OP is asking, but for a while the Hulk reverted to his early grey form and went under the name Mr. Fixit. The last I heard Bruce Banner suffered from multiple personalities, which accounted for the grey and green forms. In addition to the mentioned Rick Jones identities, he once temporarily absorbed some excess gamma radiation and became a second Hulk.
Crazy Jane of DC’s Doom Patrol had 64 different personalities, each of which had a distinct superhuman power and name.
Marvel’s Starhawk of Guardians of the Galaxy was merged with his wife Alatea and would change back and forth between the two forms.
Speaking of MPD, there’s also Bob Reynolds, otherwise known as the Sentry/Void. They sit on the couch and watch television with the power of a million billion exploding suns.
Carol Danvers has been known as Ms. Marvel, Binary and Warbird.
Sharon Ventura has been known as Ms. Marvel and She-Thing.
Jean Grey has adventured under her own name and Marvel Girl, and god knows if she ever operated herself under Phoenix. I’ve given up trying to figure that mess out.
There was an alternative universe story where Frank Castle was recruited as a replacement Captain America, but was also The Punisher at the same time.
Was this a retcon, or always part of history?
How about Jimmy Olsen? He was all the following superheroes:
Giant Turtle Boy
Elastic Lad
Flamebird
Speed Demon
The Human Porcupine
He did appear as Elastic Lad and Flamebird at around the same time, so would show up as either as the story required.
Lois Lane also went through a bunch of superpowers, starting as a witch in Lois Lane Comics #1, and including “Leopard Girl of the Jungle,” “Lois Lane, Girl Atlas” (super strength), Batwoman, The Kryptonite Girl, Super Lois (various versions), a superintelligent freak with a giant brain, a centaur, a demon, and a giant.
It was a ( I think ) one shot where the dial was changed so you dialed VILLAIN, with a big V in the middle.
Captain America’s WWII sidekick Bucky had a later career as the Commies’ Winter Soldier.
All the answers I came to give have been covered, I think. 
There was a pretty good Incredible Hulk annual where Bruce Banner became Captain Universe and fought against the Hulk.
freekalette, try to find old Moon Knight comics from the late 1970s and early 1980s, written by Doug Moench. See the wikipedia entry for a list of magazines where he appeared before getting his own title.
Stretching the concept a bit, Grimjack’s real name was John Gaunt. He died and came back in a clone body and was at least informally referred to as Clonejack. Then he met an alleged future version of himself who was informally known as Futurejack. Finally, he reincarnated and came back with the name James Twilley.
When one-third of Triplicate Girl was killed, she began operating under the name Duo Damsel.
PS238 has a time-traveling shapeshifter called Paradox. I don’t know much about him yet (his reveal was pretty recent, and there may be more about him in some back issues I don’t have yet), but I know that he has posed as at least one other time-traveling hero (or hero-in-training, at any rate) and possibly other timewalkers. I forget the exact phrasing, but I believe his claim was that he was whoever he needed to be. It seems likely that he can be multiple people in multiple places at the same time, but I haven’t seen him do so.
The Marvel character Copycat was a metamorph who could duplicate the powers of the people she copied. I haven’t actually read the stories with her in them (I just remembered the name and looked her up), so I don’t know if she tried posing as multiple characters at once. She seems to have been more of an infiltrator type. She’s currently deceased, which is probably just as well, since her abilities could readily be warped into insane levels of overkill by the wrong author.
He started with access to 10 alien forms. Various incidents resulted in him learning how to change into more different types, and a future version of him was referred to as “Ben 10K” (or something like that), implying that he had access to as many as 10,000 forms. At any rate, the adult Ben had enough different forms that he had stopped naming them.
I have no idea where his abilities stand in the new Ben 10: Alien Force series, since I haven’t watched any of it.
Booster Gold (and a bunch of his relatives) took on the mantle of Supernova a while back…
Yes? Justice Experience was introduced in the Chase series, Cam’s father was a member, and they showed the Bronze Wraith in the flashback. The group was active in the 60s before they were all apparently killed.
In the Martian Manhunter series, John revealed to Chase that he had been the Wraith, and had been captured in a trap and unable to save the lives of the other members, including her father.
So it is history according to New Earth, but it is a retcon of the 60s to explain what was going on with super heroes after the JSA retired and before the advent of Superman and the modern age.
Also, J’onn J’onzz acted as the mystical hero Bloodwynd while under the influence of a demon. I can’t remember if he also appeared as himself at the same time.
Too many to give individual shout-outs, but I do thank everyone for their contributions!
My best friend just bought Moon Knight, so I’ll have to steal his copies and read them ASAP.
My oldest son watches Ben 10, but I’d never thought of him as an option. (I narrowly was only looking at regular comics.) I’ll have to start watching the series, as the commercials I’ve seen looked okay.
Read some of Doom Patrol and hated it, but have decided to give it another go. Perhaps my tastes will have changed.
You guys are the best, and I appreciate it!
I hope so. The Morrison run is one of the most amazing runs on a comic ever. The Pollack run immediately following is ood too, but all of my Pollack issues have vanished mysteriously.
Anyone interested in Badger, he’s on the cover of this week’s issue of Madison’s alt-weekly paper, Isthmus. Associated article, about the development of the Madison comics scene, found here but without the cover illustration. If anyone would like a copy of the cover with or without the story, PM me an address. But do it quick; the issue is only on the stands until Wednesday.
Did he ever dial H-E-R? That would be interesting.