You need to do this in two steps. First add the position node from the other node and then remove the position node that was already there.
declare @PK1 varchar(11);
declare @PK2 varchar(11);
set @PK1 = 'cos190101-1';
set @PK2 = 'cos190101-2';
-- Add the new position node after the one that already exists
update T
set X.modify('insert sql:column("Pos") after (ViewcenterLayout/viewcenter_config/data/position)[1]')
from (
select T.X.query('ViewcenterLayout/viewcenter_config/data/position') as Pos,
case T.PK when @PK1 then @PK2 else @PK1 end as PK
from T
where T.PK in (@PK1, @PK2)
) as S
where T.PK = S.PK;
-- Delete the first position node
update T
set X.modify('delete (ViewcenterLayout/viewcenter_config/data/position)[1]')
where T.PK in (@PK1, @PK2);