Are there any comic book heroes or villains who've never altered their costume

No, the Big Red Cheese has had at least two slightly different costumes. The original had a flap with buttons on the chest. The 70’s and 80’s he wore a red one piece. I believe the flap came back in the 90’s.

Thank you for calling him Captain Marvel and not Shazam.

I remember once when Hulk turned intelligent, he wore this labcoat over his shredded purple pants. There was a scene where he went into the shower. When he went out, he opened a closet full of shredded purple pants on hangers, to get a clean pair.

Underdog.

He’s also gone back and forth on having a flamboyant yellow sash vs. a little yellow belt. The panel in the upper right corner of this pageshows a couple versions of him side-by-side.

It does have pockets. Who knew?

Galactus?

In his first appearance he had a big goofy G on his chest, it was dropped pretty much immediately IIRC.

Thor?

He used to not have any sleeves, now he has chain mail on his arms and legs.

**Silver Surfer

The Watcher**

http://geek-news.mtv.com/2011/05/06/the-many-looks-of-thor/

Supervillains change their costumes often to keep Paul Gambi in business.

Hasn’t the Fantastic Four uniform stayed pretty consistent?

Nope. If you Google Barry Allen and check the images,
you’ll see that he originally had and later lost the little wings on his boots and that the exact design of his lightning bolt “belt” has varied. (I think it was straight across at first, but dips in the middle now similar to Wally’s.)

Nope – they recently had all-white uniforms.

http://shelf-life.ew.com/2011/02/10/spider-man-fantastic-four-future-foundation/

http://geek-news.mtv.com/2011/02/24/the-fantastic-fours-costume-changes-over-the-years/

Nope. Also:

http://scans-daily.dreamwidth.org/1641659.html

I don’t think Plastic Man ever changed the style of his get-up

Or The Spectre

F not

I don’t know how long it lasted or if was even in the actual book, but the Specter had a blue cape and hood on the cover of More Fun Comics #52 (his first appearance).
Other than that, Spectre’s and Plastic Man’s costumes have been very consistent, if not completely unchanging.

Seems to me that Silver Surfer has sometimes been drawn with a smooth, seamless look and at other times he has visible “trunks” as part of his look.

It’s not a costume change – we’re talkng about, at most, the addition or deletion of a line. It seems to be more in how the artist thought the indication of the look was supposed to be. And maybe about whether they’d run afoul of the Comics Code Authority.