(def testds {:e [{:c :blah :x 1 :y 2}
            {:c :foo :x 2 :y 5}]
        :e1 [{:c :blah :x 1 :z 2}
             {:c :foo :x 5 :z 12}]})

=> (assoc-in t [:e 0] {:c :doo :x 1 :y 5})
{:e [{:c :doo, :x 1, :y 5} {:c :foo, :x 2, :y 5}], :e1 [{:c :blah, :x 1, :z 2} {:c :foo, :x 5, :z 12}]}
;; yay!

=> (assoc-in t [:e 1] {:c :bing :x 1 :y 5})
{:e [{:c :blah, :x 1, :y 2} {:c :bing, :x 1, :y 5}], :e1 [{:c :blah, :x 1, :z 2} {:c :foo, :x 5, :z 12}]}
;; oops :-(