Will reinstall macOS delete the current OS?
Yes and no 
The update will install new things and modify existing ones. Your data and all your programs should stay the same though, but depending on your update some programs (for example 32-bit) will stop working.

You can't just downgrade to/simultaneously use the older OS if thats what you'd be trying to do after upgrading.

Yet, with a TimeMachine backup or a clean install you can get the older OS or the OS your computer shipped with.