If it gives you errors, I can't really see a way around it. Perhaps you could copy them to a removable disk and then copy them back again, or if that is not fiesable, create a directory under /home, copy them there temporarily and then move them back across, or perhaps utilise that empty 20GB partition there.
Using Arch, I personally create a seperate partition for the cache directory (/var/cache/pacman/pkg). Then I mount this partition under the correct folder. This saves loads of time when reinstalling, which I do loads, because I often get bored and trash my system (last reinstall was this morning
). Whenever I format, I just avoid formatting this partition. Works every time.