Is it possible to update VirtualBox offline, while preserving the already installed virtual machines and their settings?
Virtualbox's updates have always been offline & always preserved the existing VMs & settings. (Atleast, since the time I started using - ~v2.0 onward I believe).
If you're running an out-of-date-version, VirtualBox may prompt you to download an update - most probably from their download page. Running the upgrade will do an in-place upgrade. The user data - ie, VMs, settings, Virtual disks, images, snapshots will not be touched.