Yes it is possible and yes people have done it before. Reasons are usually personal (family, etc) from what I understand. They need to be extenuating circumstances because it is very hard to do and isn't something you should plan on working out.
And generally, the only time you can transfer is between second and third year, and only if there is an opening in the class (someone dropped out or took a year off).
Usually only under extenuating circumstances (if they even accept transfers at all). The above reasons stated are usually the situations. and it's usually at a breakpoint - between 2nd and 3rd year.