SQL replace value in a field in a row if it exists, else create a row and populate that value - sql

Question

My request is a little strange and specific, please bear with me.
I have an Access 2003 database with a Comments table that has several hundred thousand lines in it. There are several fields - let's call them A, B, C, D and comment. Columns A-C form the PK of the table. Thus, you could have 5, 10, or 50 rows which have different comments, but all pertain to the A-C PK.
I have another table called CORE which contains a table core that has fields A, B, C, D (A-C PK) and comment_value_insert.
What I want to do:
In Comments, for each "set" of identical A-C rows (Field D can differ), if none of their comment values matches a certain value (let's call it critical), then look up the same row in CORE and insert into the comments table a new row with A-D and Comments.comment = core.comment_value_insert.
I'm not even sure if this is possible with SQL (perhaps VBA? Hence the tag). Any advice anyone?
For further clarification please ask away.
Huge thanks in advance.
EDIT
To demonstrate exactly what I want:
If my Comments table looks like
A | B | C | D | Comment
1 2 3 b val1
1 2 3 x val2
1 2 3 a val3
2 9 8 z val4
2 9 8 a val5
3 3 3 x val6
END
And my CORE Table looks like
A | B | C | D | Comment_value_insert
1 2 3 u critical1
2 9 8 t critical2
4 8 6 x critical3
END
Then I'd want the critical1 row from CORE to be inserted into Comments, as well as the critical2 row, but not the critical3 row.
Then the final product in the Comments table will be
A | B | C | D | Comment
1 2 3 b val1
1 2 3 x val2
1 2 3 a val3
1 2 3 u critical1
2 9 8 z val4
2 9 8 a val5
2 9 8 t critical2
3 3 3 x val6
END

Answer 1

Try something like this for the INSERT part. For the UPDATE part you should provide more information.
-- EDIT --
INSERT INTO Comments ( A, B, C, D, Comment )
SELECT DISTINCT C.A, C.B, C.C, C.D, C.comment_value_insert
FROM Core AS C INNER JOIN Comments as C1 ON C.A = C1.A and C.B = C1.B and C.C = C1.C
WHERE NOT EXISTS (SELECT * FROM Comments AS O WHERE O.Comment = "Critical" AND O.A = C.A AND O.B = C.B AND O.C = C.C);
Regards,

Answer 2

** This version worked for me **
INSERT INTO comments
SELECT a,
b,
c,
d,
comment_value_insert AS comment
FROM core
WHERE EXISTS (SELECT comments.*
FROM comments
WHERE core.c = comments.c
AND core.b = comments.b
AND core.a = comments.a)
AND NOT EXISTS (SELECT comments.*
FROM comments
WHERE core.c = comments.c
AND core.b = comments.b
AND core.a = comments.a
AND core.comment_value_insert = comments.comment)