Serum infusion, beginning in five, four, three, two, one.
As the cold fluid began to rush through him, Steve lost focus on the doctors and equipment surrounding him. The super soldier serum moved like an ice flow through his veins. Coming back to himself as the chamber doors closed around him, he heard a muffled, "Steven, can you hear me?" from just outside. Realizing he really should’ve used the toilet before starting, he said something to that effect, right before being blinded by a bright flash.
The light intensified and seemed to sink into him, causing his serum infused body to rapidly warm. Steve faded in and out of consciousness, disconnected from the increasing pain even as his body screamed. His bones crunched, shrinking as they condensed. Muscles tore and rearranged while new fibers grew.
Outside the chamber, the observing scientists noted the change as the agonized cries steadily rose in pitch and intensified. Shouting and scrambling to abort the experiment, they were stopped by a high voice calling out from within the chamber.
“No…Don’t…I can do this!” It was all he managed to get out. As the torture reached its crescendo Steve felt an intense tearing sensation from his crotch and the agony that was finally beginning to break through his mental fog abruptly ended. The jagged, burning flow of serum through his rapidly changing body settled into a warm soothing pulse. With the bone and muscle finalizing, buttery soft fatty tissue began to reform in a layer all over Steve’s body, some areas receiving more than others. Not that he (she?) was aware of any of this. Moving from blind pain to blind pleasure, Steve's screams started anew, no longer cries of agony, these were cries of intense, overwhelming ecstasy. The mostly male, mostly single group of scientists looked to each other with odd questioning glances, but at least a few of them recognized those sounds.
Finally, the cries slowed, quieting into barely conscious moans, and the chamber doors opened, exposing a very new, very different, extremely female Steven Rogers.
"What is this?" barked a gruff voice from a man in general's uniform. “Years of work and endless resources and this is what you give me? A damn pinup girl?! Doctor, I expect you to salvage this, or you can consider your stay in America canceled!“
“General, what should we do with Rogers?” asked another soldier.
"Well, she’s easy on the eyes at least. Get her a uniform and put her in the damn USO show. Private Rogers will be an inspiration to the troops, one way or another."