How to change SRID of geometry column?
You should use the ST_Transform function. Also use the function AddGeometryColumn to create your new column, to ensure all the necessary constraints are also created:
SELECT AddGeometryColumn('table','the_geom4258',4258, 'POLYGON', 2);UPDATE table SET the_geom4258 = ST_Transform(the_geom,4258);ST_SetSRID just sets the projection identifier, but does not actually transform the geometries.