Did you do this online?
Yes, I was able to do it online for Nebraska. I think I just googled “how to update voter registration for Nebraska” or along that line, and it took me to the site. I had to provide identifying info (that other people would be very unlikely to have), and then it let me make the change.
But it might be better to register as a Democrat, because then you might be able to get multiple ballots more easily, for future elections.