Several servers converted renewal content to pre-renewal, it's not that hard. The only tricky part is converting the drops, because you obviously don't want renewal equips to drop and you don't want to have the ridiculously high elunium and oridecon drop rates that came with some renewal maps, but of course you still want to have drops on the maps that are worth hunting for.
For monster stats you usually want to reduce their level and increase their ATK and base exp (but depends on level range of monsters), but that's easy to convert.
And of course you need to move all the NPCs from renewal to pre-renewal (including warp), but that should also not be too hard, just a bit of busy work (especially since you also need to test it all).