That seems quite a bit more complicated than this:

CREATE PROCEDURE [dbo].[setTodoSequence]

(

@personID int,

@todoID as int,

@todoSequence as smallint

)

AS

SET NOCOUNT ON;

declare @oldTodoSequence smallint

declare @direction smallint

declare @start smallint

declare @end smallint

-- get the old sequence

set @oldTodoSequence = (select todoSequence from dbo.ToDoList where todoID = @todoID);

SET @direction = Sign(@todosequence - @oldtoDoSequence)

SELECT @start = CASE WHEN @direction = -1 then @todosequence ELSE @oldtodosequence END

, @end = CASE WHEN @direction = -1 then @oldtodosequence ELSE @todosequence END

update todolist

set todosequence = CASE WHEN todoid = @todoID THEN @todoSequence ELSE todosequence - @direction END

where todosequence between @start and @end

and personId = @personid