Converting Between Clojure Maps and Lists
Here's a few other ways you can go about it:

(defn map-vals [f a-map]
  (zipmap (keys a-map) (map f (vals a-map))))

(defn map-vals [f a-map]
  (reduce-kv #(assoc %1 %2 (f %3)) {} a-map))


or if you're set on transients/into

(defn map-vals [f a-map]
  (persistent! (reduce-kv #(assoc! %1 %2 (f %3)) (transient {}) a-map)))


This is a use case that could maybe stand to have a more elegant core tool, though. Or at least, I'm not the only one to miss it.
For what it's worth, the zipmap approach actually appears to perform best in my cursory testing and is pretty clear in its intent to change the values while keeping the keys as they were. If you're using a function literal it also means that fn doesn't have to take keys it's not going to do anything interesting with, which could make it a little clearer.