Cycloneium, that is not always a real option, seeing as Ultimate is still 1.4.7 and DW20v2 is 1.5.7. There were several improvements in base Minecraft in the version upgrade and several mods that do not have a 1.4.7 version or have a 1.4.7 version that are quite different or less developed than the 1.5 versions. For example Tinker's Construct and Natura by mDiyo. Infinicraft (the old version) existed in 1.4.7, but it did not give the same feeling as the 1.5 versions give.
I also have never done the transfer either, but I would suggest if you are really determined to do so that you make a list of the mods that ultimate has that DW20v2 does not have (ExtraBiomesXL, SecretRooms, Gregtech if you want that, ect...) and get the configs as close as you can in terms of Ids and energy settings so that they don't mess with existing infrastructure. I don't think you would need to delete any files from the world save, but I'm not 100% sure. The kind of work involved basically negates the reason behind using a modpack for most people. I would honestly suggest you just start from new if that is a viable option. Several of the mods in DW20v2 give some good early game content so you could miss out on quite a bit by skipping to mid/endgame.
