[m_kock]

Is there a way to change users in Plants vs Zombies?

[Paul Acevedo]

You can't change users in Xbox Live games. A single GamerTag is tied to the phone and all Achievements/Xbox Live will be associated with that GamerTag.
There is one XBL game that lets other players input their names on the high score board, though - Doodle Jump. But it's not a common feature among Xbox Live games.