In mid-last-century Alabama, no one said "I swan".

What was said was, "I swa-nee", meaning "I do declare", perhaps, as tsuwm reported, deriving in sound and meaning from "I shall warrant".

Wouldn't you think that in order for an euphemism to be a euphemism, a conscious avoidance usage would be the determinate aspect?

Back then and there a sentence like this would not be unusual...

"Well I swanee, would you look at that youn'un, I swear he's the spittin' image of his uncle Harry."