From time to time the system changes major version and we should rebuild (or update) many components, placing them into a new repo. Let say current version is 2 (du2) and new one will be du3. First of all you should update /etc/apt/sources.list like this:
deb http://apt-mirror.loc/dilos dg3-unstable main contrib non-free deb http://apt-mirror.loc/dilos du3-unstable main contrib non-free deb http://apt-mirror.loc/dilos dg2-unstable main contrib non-free deb http://apt-mirror.loc/dilos du2-unstable main contrib non-free deb-src http://mirrors.kernel.org/debian/ buster-updates main contrib non-free deb-src http://mirrors.kernel.org/debian/ buster-proposed-updates main contrib non-free deb-src http://mirrors.kernel.org/debian/ buster-backports main contrib non-free deb-src http://mirrors.kernel.org/debian/ buster main contrib non-free
Here we placed new repo above old repo, when you rebuild a component it has a new version, that means a higher priority to install it. Due to transiotion rebuild you use old repo to install build dependecies and use buster source repository from the debian upstream.