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Switching data between two columns
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The analogy given assumes that
1) there is a primary key in the table
2) the affected columns are not in the primary key
both of which are fulfilled by the OP's schema.

Also note the non-optional ellipses which represent the remaining fields of the table and should not be empty. It was not intended to be executed instead of an UPDATE, just as a mental model.

This works for your schema, however:

asql> select rowid,* from xx;
rowid       c3  c4
----------  --  --
1           1   2
2           3   4
asql> replace into xx (rowid,c3,c4) select rowid,c4,c3 from xx;
rows inserted
-------------
2
asql> select rowid,* from xx;
rowid       c3  c4
----------  --  --
1           2   1
2           4   3