I used time travel to have the PCs retcon the setting. After running a campaign for a year or so, I really wasn't happy with how one of the countries was working out. It wasn't appropriate to the campaign that had developed, and I couldn't have events unfold as I'd originally intended. There's a mystical forest in the heart of the continent that had played a fairly large role in the campaign, and the PCs were some of the few that dared venture into it. Its odd and unpredictable effects were well established, so emerging six centuries in the past didn't even surprise them. A couple of chance encounters, a soldier saved, and when they return to the present the country in question is now much more suitable for the rest of the campaign.