The most straightforward way, given that the operation of moving from a varchar column to a CLOB is disallowed, would be to create a new column and move the data from the old column to the new column:
ALTER TABLE some_table ADD (foo CLOB);
UPDATE some_table SET foo = old_column;
ALTER TABLE some_table DROP COLUMN old_column;
ALTER TABLE some_table RENAME COLUMN foo TO old_column;