I don't think the case statement will work that way. Maybe something more like this?

update <table>

set <field> =

case when <field> = 0 then 1

else 2

end

Before you do that, I'd see if there was a better way to do this. If you are using a trigger, wouldn't it be better to make this change in the sproc that's making the original call to update the table?

Mark