"X1" is the original name and more convenient, both in saying it and in typing it, but it's also wrong. The only correct name is "X2010", based simply on the fact that the name was officially changed to "X2010". It's "the car formerly known as 'X1'." It's kind of like how there's no such thing as a Brontosaurus, since the name was mistakenly given to another specimen of the previously-named Apatosaurus, and upon realizing that they were the same creature the name "Brontosaurus" was dropped in favor of the name that was given first, except in the case of the X2010 it isn't the name that was given first but rather the final official name and the older is simply defunct.
Sure, we know what you mean by "X1", but it's still the wrong name.