Trail names can include almost any characters, including foreign ones. Trail names are not case sensitive, so you would not be able to create an account name such as "green tortuga" since "Green Tortuga" is already taken.

All trail names must include at least one Latin-based letter or number.

And, of course, you cannot use a trail name that someone else has already registered nor many variations of such a trail name.

You can change your trail name from the Account Info page. The only effect this has is that your trail name changes—you still have all of your plants, finds, message board posts, etc.—everything is the same as before, except that your trail name is different. It's as if you had the new trail name the entire time.

Keep in mind that this is the way other letterboxers will address you at gatherings, events and whatnot. It's helpful if the name you choose is actually pronounceable.