What did Baghdad's translators do to Greek philosophy?

Preserved it, corrected it and extended it. The translation movement under the Abbasids from the eighth century rendered Greek scientific and philosophical texts into Arabic, often via Syriac, funded generously by caliphs and private patrons - and the translators were frequently Christians working for Muslim employers on pagan texts.

Much of Aristotle reached medieval Europe through that route, translated from Arabic into Latin in Toledo and Sicily, sometimes with the Arabic commentaries attached. Averroes was so central to how Europe read Aristotle that scholastic writers refer to him simply as the Commentator, and Aquinas is arguing with him constantly.

Calling it preservation understates it, which is the usual failing of the phrase. Ibn al-Haytham rebuilt optics on experiment and argued that vision works by light entering the eye rather than leaving it; al-Khwarizmi gave algebra its name and method; Ibn Sina's Canon was Europe's medical textbook for five hundred years. This was a research culture rather than a library, and the framing of it as a holding operation for European ideas is a piece of nineteenth-century historiography that has been hard to dislodge.