Honest question since I'm not a comic book guy: have there been instances or incarnations of Captain America where he's not Steve Rogers?
Just an addendum to what's already been posted: there have been many Cap substitutes over the years.
When Steve Rogers fell into suspended animation at the end of WWII, his suit was filled by Jeff Mace (a Golden Age superhero formerly known as the
Patriot) and then, when Mace was killed, the role was taken by William Naslund (a hero formerly known as
The Spirit of 76).
A retcon was made to explain why, in real life, Captain America comics had the subtitle "Commie Smasher!" during the 1950's. Steve Rogers was still in suspended animation, but a man who hero-worshipped Cap created a version of the super-soldier serum & injected himself (and a youthful partner) to become the Cap & Bucky of the 1950's. (So, while in real life, Cap & Bucky were still appearing, decades later it was revealed readers of that era had been following characters who'd taken the heroes' places.)
But the serum drove the fake Cap & Bucky mad with anti-Communist sentiment - they'd ultimately be put in suspended animation themselves only to decades-later return to menace the real Captain America who was found floating in ice and revived by the Avengers in the 1960's.
This hardly includes all those who've either stolen Cap's name & costume and were imposters for a single story or who have substituted for him due to his apparent retirement, death, or with his blessings.