So, I'd like to get the new 18.04 and I want to ask you guys this: is it safer for me to reinstall Ubuntu or can I just use the update manager with the "do-release-upgrade" without problems?
Always better to do a new install than an install by upgrade.
If you have a lot of applications downloaded, never fear. The easiest way to retain them is first to make a text file of all apps on your system -
sudo dpkg --get-selections > MyApps.txt. Keep this file in your Home folder, which should be backed up. Then download and install the new release.
To retrieve your apps -
sudo dpkg --get-selections < app-backup-list.txt,
sudo apt-get update and
sudo apt-get upgrade