Can't I just update my 1.6.2 and 1.6.4 mods to the 1.7.10 version or 1.8 version? Will that erase any of my mod items I had in my inventory and in my chests?
No it's a case of old Forge having an unknown bug that was revealed recently. Fixed long ago by Forge team.
In short: Unless a mod is specifically designed to be upgradeable from 1.6.4 to 1.7.10, migrating the world will break it. And there are very, very few mods that are designed to handle that upgrade.
"Break it" in this case depends on way too many factors to predict but can range from "stuff disappearing", through "world turning weird", to "crash on load".
So how do I fix it? Just download the java fixer? Do I need to uninstall my Java versions too when I download the java fixer?
I also have the java update 8 so I believe that may have been an issue with some forge mods
http://www.minecraftforum.net/forums/mapping-and-modding/minecraft-mods/2206446-forge-1-6-4-1-7-2-java-8-compatibility-patch
Can't I just update my 1.6.2 and 1.6.4 mods to the 1.7.10 version or 1.8 version? Will that erase any of my mod items I had in my inventory and in my chests?
So how do I fix it? Just download the java fixer? Do I need to uninstall my Java versions too when I download the java fixer?
Edit: I am confused of how to use the legacyJavafixer file when I download it...Is there any step by step instructions you guys can give?