Story about a man whose mind is transported to the future into another person's body

I can't seem to find a story I read sometime in the past – space opera / time travel.

The plot goes something like this: a man's mind is transported to the future where he finds himself in another's body, pretends to be him, then fights in a war, and is ultimately transported back to his own time.

The book has an apparent sequel, where the protagonist is transported again to the future in the same manner. I also seem to remember the second story retconning the ending of the first slightly.