The Wormhole wrote:
As I understand it, when originally cast no one informed Jessica Steen that the role was going to be a regular. At the time, Lost City was believed to be SG-1's final episode. When she learned the character was meant to be a regular on Atlantis, she was unable/unwilling to commit to a series and so the role of Dr. Weir was recast.
Not true, quite the opposite. After the casting change her agent actually commented that they don't understand why the change was made, and they'd actually paid her not to take more work so she'd be available for Atlantis. As with most things, it was probably a money issue. She was quite a bit higher profile than Torri Higginson at the time.