I can see her agreeing to do the reensouling spell, but she didn't know she'd have to do the Delothrian's Arrow spell to break the jar, and she definitely didn't know she'd get into a battle of wills with Evil Cordy. That's significantly more magic - and riskier magic - than she bargained for, yet she's totally not worried. She actually goes black-eyed during it and doesn't even show a bit of concern. That seems bizarre.