The situation is the following: I need to downgrade the boost package from the version 1.56 to 1.55. The dedicated page on the arch wiki suggest several solutions. The one I chose is using the AUR package downgrader. Its usage is really intuitive:
This is an interactive tool. For each package, a simple text menu is printed with all availables versions for the package. Just choose the one you want. If the wanted package is not in your cache (/var/cache/pacman/pkg/), it will be donwloaded and put into your cache.
Note that it can only proceed one package at a time.
For the specific usage, it cannot downgrade both boost-libs and boost at the same time, and thus return an error. I installed both package one by one by hand with pacman -U <path_to_package>. I made it twice, the first one accepting the unresolved dependencies, the second one checking all is OK.