well, it can be argued both ways... hard to say...
Some can argue that the first sword was merely a manifastation of Rukia's power, and that the second one was brought in by Ichigo's own shinigami's powers...
And others can argue that it has always been the same zanpakutou...
There's enough evidence to support both versions... I preffer the first one... For one thing... how come did the cover (or however it is called) from the first sword disappeared when the new sword came out...
And also, how come the handle also disappeared once the new sword came out? From all the shikais we have seen, I don't remember seeing any of those two change...