user> (def data  {:x 110 ::old {:x 100}})
#'user/data
user> (update-in data [::old] merge {:x 110 :z 5})
{:user/old {:z 5, :x 110}, :x 110}
user> (-> data
          (update-in [::old] merge {:x 110 :z 5})
          (merge {:x 200}))
{:user/old {:z 5, :x 110}, :x 200}
user> (defn update-keep-old
        [m k v]
        (let [ov (m k)]
          (-> m
              (assoc k v)
              (update-in [::old] k ov))))