iterate through Postgres enum values?

I have a Postgres enumerated type used to describe the node class (order sensitive) of a source/destination relationship table. Question is, how can I dynamically fetch the next value (if any) of an enumeration instance?

CREATE TYPE MyEnum AS ENUM ('level1', 'level2', 'level3');

SELECT dst_type, dst_id
FROM my_relationships
WHERE src_type::MyEnum = next(dst_type::MyEnum)      -- does not work
-- WHERE src_type::MyEnum = dst_type::MyEnum + 1     -- does not work
AND src_id = 'd4fc1d5d-b054-d37c-4e80-2c04a6b3dbf8';