I know a few people, including myself, who have had this issue. When selecting an older version of New World Mod Pack (7.5 in my case), the FTB client would launch an unmodded 1.6.4 instead. Looking in the versions folder of FTB, only the 1.6.4 jar was being loaded, despite the client correctly downloading 1.5.2 mods. Until this issue is fixed, here is a workaround:
BLUF: Move the mods and saves from NWMP to Unleashed and launch "Unleashed" with NWMP mods/saves
Select a modpack that is still 1.5.2 in it's latest release. I chose Unleashed and will be using it as the example. Force update or install a fresh copy of Unleashed. Make sure the modpack runs fine before continuing. Navigate to the Unleashed folder (/FTB/Unleashed). In the minecraft folder, delete the following folders: config, coremods, mods, saves (copy a backup if you have worlds saved for unleashed), and stats. This will make Unleashed a moddless 1.5.2 shell.
Navigate to the New World Modpack (/FTB/NewWorldModPack) and open the minecraft folder again. Copy the following folders: config, coremods, hats, mods, resources, saves, and stats. Paste these folders into the Unleashed folder you deleted the previous folders from.
Launch the FTB client. Select Unleashed and press launch. It should not ask to update the modpack since as far as the client knows it is up to date. If for some reason it does, do not update. The "Unleashed" modpack will load with all the New World mods and your saved worlds in 1.5.2.
This should only be a short term problem, but I wanted to put this out there since it took me two days to discover the client was launching the 1.6.4 jar no matter what. If you need clarification on anything, post here and I will try to get back to you.
BLUF: Move the mods and saves from NWMP to Unleashed and launch "Unleashed" with NWMP mods/saves
Select a modpack that is still 1.5.2 in it's latest release. I chose Unleashed and will be using it as the example. Force update or install a fresh copy of Unleashed. Make sure the modpack runs fine before continuing. Navigate to the Unleashed folder (/FTB/Unleashed). In the minecraft folder, delete the following folders: config, coremods, mods, saves (copy a backup if you have worlds saved for unleashed), and stats. This will make Unleashed a moddless 1.5.2 shell.
Navigate to the New World Modpack (/FTB/NewWorldModPack) and open the minecraft folder again. Copy the following folders: config, coremods, hats, mods, resources, saves, and stats. Paste these folders into the Unleashed folder you deleted the previous folders from.
Launch the FTB client. Select Unleashed and press launch. It should not ask to update the modpack since as far as the client knows it is up to date. If for some reason it does, do not update. The "Unleashed" modpack will load with all the New World mods and your saved worlds in 1.5.2.
This should only be a short term problem, but I wanted to put this out there since it took me two days to discover the client was launching the 1.6.4 jar no matter what. If you need clarification on anything, post here and I will try to get back to you.