Backup
If the first step in any procedure is ‘backup your data’, for goodness’ sake do as it says!
Don’t think that because you’re more technical/brighter/don’t really care about the existing data that you’ll be OK.
If you don’t have a back up Murphy’s Law dictates that something disastrous will happen and you’ll lose everything.
If you do have a backup, by the same logic you’ll be fine. Think of it as an insurance policy 