(asdf-readtables:defsystem/readtable :foo :readtable :foo ...) ==> (ASDF/DEFSYSTEM:DEFSYSTEM :FOO :DEFSYSTEM-DEPENDS-ON (:NAMED-READTABLES) :CLASS ASDF-READTABLES:SYSTEM/READTABLE :READTABLE :FOO ...)
(change-class (asdf:find-system :bar) 'asdf-readtables:system/readtable :readtable :bar)
(change-class (asdf:find-system :bar) 'asdf:system)