With a good solid group of playtesters you can hash out major quirks pretty easily. The foundation has already been set for what the game is about and it is easy enough to see what is broken or overpowered.
Granted when it comes to old codexes we've played them to death and can tell what excels and what doesnt but i still dont think thats enough to modernise it for now. Obviously if your talking recent fallen incarnations such as tau, eldar, chaos, or the most recent orks then it'd be ok. These codexes still correlate to the core rules we use now. Any older than that and you'd have to expend so much time reverse engineering the rules that i think it wouldnt be worth it in the long run.