Its not really possible to switch modpacks on an existing map.
However, one can always try... if you don't mind potentially loosing lots of stuff. The success will all be determined by how many mods the two packs have in common. Oh, and Minecraft versions. Because a lot of mods don't support the conversion of 1.6.4 to 1.7.10 anyway.
My reading is that TW2 is for minecraft 1.6.4? And Infinity if 1.7.10.. so this is a worst case situation. How desperate are you?
Stage 1 is pre-preparation
You need a minecraft client with 3 profiles - your current world and TW20 pack as a backup. An intermediate profile for mc 1.6.4 that only has the TW20 mods that exist also in infinity, and a profile with the Infinity pack for MC 1.7.10,
Stage 2 is clean up
You want to remove all content that is not going to be in Infinity because the mods no longer exist. This mod
http://www.minecraftforum.net/forum...05-world-generation-manager-v0-4-4-ungenerate will be your friend.
In the copy of your world in the TW profile, remove as much content from the mods you know are going to be deleted. Ungenerate them with the above tool.
Copy the world to the intermediate profile and load it and see if the ungeneration was successful or if big holes have appeared. Wander around.
You now have a copy of the game that should not contain any references to blocks or entities that cannot be in Infinity
Stage 3, convert from 1.6.4 "TW intermediate" to 1.7.10 Infinity:
Copy the save game from the intermediate profile to the 1.7.10 Infinity profile. And load it up. If it crashes, or the error log fills with error spam from a mod, go back to stage 2, and remove the offending mods content and the mod itself.
If it loads up, check out the world. Perhaps excavate some chunks to see if there are pockets in the ground where there should be blocks. If there are - and you can figure out what should have been there, go back to step 2, and ungenerate that content.
You now have a copy of the game, for 1.7.10, with all the blocks that could be converted.
Stage 4, regeneration
Now, your world will be missing a lot of worldgen for the mods that were added. So, make a backup. And use the above mod to "re"generate the mods content. Or use COFHWorld's configuration to generate missing ores. Some mods - like Thaumcraft - have their own regeneration support.
If you are trying to preserve just your core base, and regenerate the surrounding terrain, an alternative is to identify the region file(s) that contain your content, and delete the other ones so they can be regenerated from scratch with the new infinity mods.
Caveats:
* Given the changes in biome adding mods, any new terrain generated after the conversion is likley to have harsh edges where biomes don't match up.
* Very few mods had the same block set going from 1.6.4 to 1.7.10, in no small part because forge changed from block id's to a text name representation for blocks. For the mods that supported block id upgrading, there was still a staged set of updates that needed to be applied in order to migrate existing blocks from the old to new registration.