You might be able to, but I think it would be such a disjointed experience. As Zeth mentions you will have problems with ore and rocks that wont spawn in earlier visited areas. You will probably also have problems with new biomes and features not spawning and if they do integration with the old will not be pretty.
You will also not get any of the new world gen that is part of Monster(ATG, BoP(more than the 5 biomes of DW20) and Highlands).
I would recommend keeping the DW20 pack for a while yet and then when Monster (hopefully) soon gets to stable make the switch over to a new fresh Monster server.