Be ready to point out some lose-lose conditions on why you don't want to go there, or reasons why you'd be a poor choice.
I've found that with work travel, there's no viable way to say I don't want to travel to destination Y when I was willing to travel to destination X just because I dislike Y and like X. For example, I love cities and would travel to just about any US city. But I really don't like most suburbs. I can't easily say "No" to Ithica, NY, and "Yes" to Washington, DC where both are in the same country, speak the same language, and have the same basic culture...
However, having applied to and being accepted in a job where you were fluent in the language of the location you thought you'd be going to, and you are NOT fluent in the language of the new destination is a viable reason. It was more valuable for the company to send you to that first location, and more reasonable for you to go.
Similarly, travelling to a peaceful, safe location vs. travelling to a war-torn, highly dangerous location, are generally considered reasonably large differences - and some jobs will even offer hazard pay for the latter circumstance.
The general reasonable differences are safety, capability to travel within the location and work in an effective way, and the right to expect a reasonable trade-off - for example, if the second location is 20 hours farther away, it reasonable to expect more accommodation for travel logistics and allocated time to recover from jet lag.
It's generally harder to sell the idea that you'd rather travel to a given location because of family, friends, or personal preference for the location.
Negotiation - However, if this is changing the stakes, it's fair to offer to renegotiate. For example, traveling 2 hours away is different that traveling 20 hours. At 2 hours travel for 2 days, you're not away from home all that much. You may have been willing to drive for the convenience of having your own car. At 20 hours of travel, you have to stay for a week to make the trip a worthwhile investment of your time and their money, and that can take you away from home for 2 weeks. At that point, asking for time off, a different type of travel arrangement or other tradeoffs is fair - for example many people who do longer work travel work weekends and then take time off to be with their family after.
Fair warning - your company will want to send you where the business is. It's far easier to recruit and train people who can go where the work is, than to find work were the people will go. The ability to make money is essential for your work to make the revenue that pays your salary. It's one thing to be the best person for a certain location so that you make the company as much business benefit as possible. It's another thing to seem unwilling to go to the location where the business is unexpectedly growing. Know which case is more likely for your current situation.