Complex sentence: Some distributions allow other init systems to be used; in such distributions, only the default init system (for new installs) was changed to systemd, while switching the init system is usually possible by installing the appropriate packages.
Split into simpler sentences: Some distributions allow other init systems to be used; only the default init system (for new installs) was changed to systemd. For such distributions, switching the init system is usually possible by installing the appropriate packages.