In which case I would lose my OS X license?
The worst thing you can unintentionally do is lose data!

So, before doing anything, ensure you have a backup regime in place (such as using Time Machine). Once you know you have a backup, then the risk is limited to potentially wasting a lot of time reinstalling macOS if things go wrong and then migrating your data across from your backup.

If you do need to reinstall macOS, then you have three options you can try:


commandR to reinstall the latest macOS that was installed on your Mac, without upgrading to a later version 
optioncommandR to upgrade to the latest macOS that is compatible with your Mac
shiftoptioncommandR to reinstall the macOS that came with your Mac, or the version closest to it that is still available


But remember - you must have a backup regime in place!