update with subselect (long)

Hello all,
I'm working with a genealogy database where I try to implement a
somewhat unconventional model for names. The purpose is to allow
different naming styles, especially the old Norwegian naming style with
Given name/Patronym/Toponym instead of the Given/Patronym style that
appears as God-given by current genealogy software. To that end, I have
designed some tables in this way:

CREATE TABLE name_part_types ( -- a key/label pair
name_part_type_id INTEGER PRIMARY KEY,
name_part_type VARCHAR(50)
);

CREATE TABLE names ( -- one person can have multiple names
name_id INTEGER PRIMARY KEY,
person_id INTEGER REFERENCES persons,
is_primary BOOLEAN NOT NULL -- but one must be primary
);

CREATE TABLE name_parts ( -- a name has multiple name-parts
name_part_id INTEGER PRIMARY KEY,
name_id INTEGER REFERENCES names,
name_part_type INTEGER REFERENCES name_part_types,
name_sequence INTEGER NOT NULL, -- internal sort order of name part
name_part VARCHAR(100)
);

My name_part_types table presently looks like this:

slekta=> select * from name_part_types;
name_part_type_id | name_part_type
-------------------+----------------
1 | prefix
2 | given
3 | surname
4 | suffix
5 | patronym
6 | toponym
(6 rows)

My current genealogy program, The Master Genealogist (TMG), stores names
in the conventional pigeon-hole way, within the fields Prefix / Given /
Surname / Suffix. This form is quite awkward regarding old Norwegian
naming practice, and I have been using the Surname field mainly for
recording patronyms, and the Suffix field for toponyms (ie. "farm
names"). I've written a FoxPro to SQL conversion script (using Perl and
the XBase module) to dump the data from the TMG database. A typical
name_parts set may look like this:

slekta=> select * from name_parts where name_id = 1652;
name_part_id | name_id | name_part_type | name_sequence | name_part
--------------+---------+----------------+---------------+--------------
3643 | 1652 | 2 | 0 | Christen
3644 | 1652 | 5 | 1 | Jonsen
3645 | 1652 | 6 | 2 | Stavdal
(3 rows)

Now I'm starting to approach my point. The values (2,3,4) in the
name_part_type column should be changed to (2,5,6). As the Suffix field
in the overwhelming majority of instances is used only if the name is
on the Given / Patronym / Toponym form, I figure that it should be easy
to change the name_part_type here. Initially, I ran this update:

slekta=> update name_parts set name_part_type=6 where name_part_type=3;

So far, so good. But how do I change the name_part_type from 3 to 5 for
the names with the same name_id that were altered by the previous
command? This is my latest try:

slekta=> begin work;
BEGIN
slekta=> update name_parts set name_part_type=5
slekta-> from (select name_id where name_part_type=6) as gpt_type
slekta-> where name_id=gpt_type and name_part_type=3;
ERROR: subquery in FROM may not refer to other relations of same query
level

Ideas, anyone?
--
Leif Biberg Kristensen
http://solumslekt.org/