A growing trend among Paul scholars is to call Paul's Damascus Road experience a "call" or "commissioning" rather than a "conversion." This is helpful in that it reminds us that Paul did not move from Judaism to Christianity, but from Judaism in infancy to Judaism in full blossom ("grown-up" Judaism - the law was, after all, a pedagogue). But I am increasingly frustrated by the way this proposal often leaves behind that which is fundamental to Paul (how sinful people can be right with God, in light of both their failure to keep the law and their frequent pride whent hey do keep it) for the sake of that which is important but secondary (the horizontal concern of gentile inclusion).
So this morning I read an article on the topic by James Dunn and was thinking through the question again. He asks on the first page, "Given that the verb 'to convert' means 'to turn (round),' from what did Paul convert and to what did Paul convert?" (p. 348 of this volume)
Here's how, at this point in my development, I would answer.
FROM a Christ-less Judaism that tended (being composed, as it was, of humans) to view God's favor as that which must be earned by obedience and therefore, derivatively, to see that favor as available only to those to whom a guide to obedience had been given,
TO a Christ-climaxed gospel--Judaism brought to fruition--that realized both the failure to keep the whole law and the pride naturally engendered by those parts that were dutifully kept, and instead the utter gratuity of God's favor by virtue of the ultimate sacrifice, and therefore, derivatively, to see that favor as available to all.