About transcriptions -- Lest we forget, many composers have been known to borrow from themselves. In "Messiah", the "Lift Up Ye Gates" is almost a re-hash of a movement from one of Handel's own concerti grossi. Listen to Franck's "Symphonic Variations" and see how close some of it sounds to the slow movement of the "Symphony in D Minor". They were written not too far apart in time, but the "Symphonic Variations" was before the symphony.
And borrowing themes from elsewhere is not unheard of. How many places has the "Dies Irae" theme showed up? A few, and Berlioz' "Symphonie Fantastique" is just one.