Backups go over wifi, not mobile data, and iOS warns you if it's unable to backup (and you can always back up to iTunes if you don't want to add more space). Ma

Backups go over wifi, not mobile data, and iOS warns you if it's unable to backup (and you can always back up to iTunes if you don't want to add more space). Major OS upgrades are always a little risky no matter the platform, upgrading without having an up-to-date backup is generally a bad idea. In fact, just using an iPhone that has critical data on it without a backup is a bad idea, because you can always break your phone.