Why? Why the change? What difference does thirty years make? For me, the difference is a lot. This is something I learned from reading Harry Turtledove's World War series. For Turtledove, it was important that his aliens not be too far advanced in their technology that humans would be overwhelmed. To do this, he made his aliens some fifty years ahead of mankind in World War II, 1990s material, which was around the time he wrote the first novel In the Balance.
I like this move, and I'm not ashamed to say I've given it consideration for my own aliens. But the problem is that humanity in the mid-1800s would never be able to combat a foe with jet fighters. Yes, they had aircraft during the Civil War, but that was limited to balloons. The idea was to have the story end with America at about a World War I level. A fifty-year leap in technology seemed to jump way too many sharks for me. Put the story in the 1890s and you cut that gap in half.
Why do I bring this up? Why do I divulge any of what I know about writing? Because I'm hoping you'll learn from it and apply it to your own work. Imagine the North and the South in an epic battle with aliens wielding machine guns and missiles. That was a pretty impressive image to me when it popped in my head, but when I got to thinking about it, it wasn't realistic enough. The turn of the century? Yeah, that could still give things a nice steampunk element.
My point: whatever cool idea you got swimming in your head like a naked supermodel, put a little more thought into it. On its own, it could fall like a house of cards, but with some development, it could be the start of something fantastic.