Updating world 1.6.4 -> 1.7.10?

Hello everyone,

I've been using Forge for 1.6.4 for a while, but since some updates for a mod came out, I went and updated to a newer version for 1.7.10

Upon loading one of my worlds, all non-vanilla blocks that were previously there were gone. (I do have a backup,  because I didn't feel sure about whether or not it would work properly anyway.) I looked around the internet and read stuff about how in 1.7 a whole lot of things had been changed etc, so I assume this is the cause. Is there any way to successfully convert my world from 1.6.4 to 1.7.10?

Thanks in advance!