Loot tables are rather off and would have to be dramatically adjusted for many zones -- noobie zones in particular, as you'd have to remove a good deal of the drops as they're non-classic. Fortunately, though you might have 30 different points where a_skeleton spawns, all those generally use the exact same loot table, so you only have to change it once.
Everything is extremely easy to adjust -- it's just time consuming. The part that would need the most tweaking would be drop rates/spawn rates for some of the rarer creatures.
Another big part would be adding in drops that aren't there anymore.
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