CREATE TABLE t1(id INT IDENTITY(1,1),SortValue INT)
INSERT INTO T1(SortValue)
SELECT 1 UNION ALL
SELECT 2 UNION ALL
SELECT 3 UNION ALL
SELECT 6 UNION ALL
SELECT 7 UNION ALL
SELECT 8 UNION ALL
SELECT 9 UNION ALL
SELECT 20

declare @id int = 4
declare @change int = 2 -- 4,7
declare @group int = (select id-SortValue from T1 where SortValue=@change)

update T1 set SortValue = case id when @id then @change else SortValue+1 end
    where id=@id or id-SortValue=@group and SortValue>=@change

select * from T1