postgresql replace table without losing dependancies

CREATE TABLE a (
  id serial NOT NULL,
  -- a bunch of other columns
  CONSTRAINT a_pk PRIMARY KEY(a)
);

CREATE TABLE b (
  id serial NOT NULL,
  a_id bigint refernces a(id)
);

--REPLACE a with a new table without violating b FOREIGN KEY relations(somehow)
CREATE OR REPLACE a( 
  id serial NOT NULL,
  -- a bunch of new columns
  CONSTRAINT a_pk PRIMARY KEY(a)
);

• Disable any FKs pointing to the table

• Create table SAVE as SELECT * FROM A

• Truncate table A

• Begin to create a script with the following

with xfer as (select cast ('a' as text) as targ_tab, cast ('b' as text) as from_tab)
select xfer.*, tabs.table_name as targ_table_name, cols.table_name as from_table_name,
  'alter table ' || tabs.table_name || ' add (' ||
  coalesce(cols.column_name, '') || ' ' || coalesce (cols.data_type, '')
  ||  
      case when data_type = 'numeric' and coalesce (cols.numeric_precision,
          cols.numeric_scale) is not null then
          '(' || coalesce ('' || cols.numeric_precision, '*') || ', ' ||
          coalesce (cols.numeric_scale, 0) || ')'
      else ''
      end
  || ')' 
  as synt
from xfer join
  information_schema.tables tabs on (xfer.targ_tab = tabs.table_name) 
  join
  information_schema.columns cols on (xfer.from_tab = cols.table_name)
where not exists (
select 1
from information_schema.columns targcols
where targcols.table_name = xfer.targ_tab
and targcols.column_name = cols.column_name
)
order by ordinal_position

• Repopulate with INSERT/SELECT

• Re-enable foreign keys