Most easy classical music pieces simplified for beginners have different key signatures from the original. What are the pros and cons (especially) of playing them before playing the original scores later on?
There‘s no contra at all to simplifying a classical music piece for a beginner.
it can be an approach to music and open for someone that never came into contact with classical music if he couldn't learn to play a simplified arrangement for piano e.g. Für Elise. Also easy transcriptions for brass bands can be a door opener for classical music for people who are used to play or listen only to march music.
it can be more beneficial and motivating for a beginner to play a simplified arrangement of a Menuet of Bach or the Moonlight Sonata, the tune “home” of the “New World” symphony of Dvorak.
There are certainly more pros about it. How surprising will it be if someone who knows only a simplified version will hear for the first time the original work.
Only purists and musical sectarians could find a contra argument.
This is probably an unusual example, but one of my flute books has a version of The Swan (Saint-Saens, Carnival of the Animals) which is not only in a different key, but several tones higher. That makes it more difficult, not less. I can only imagine the writer of the book wanted to give the student more practice with the higher notes.