Installing a different OS on a current dual-boot iMac

I currently have an older iMac (2011) that can dual-boot either Lion or El Capitan.

I now want to delete Lion and install the High Sierra OS in its place (but leave El Capitan alone, so that I can still boot-up in it if I need to).

When I start the upgrade, will it ask me which volume to install High Sierra to? And if I install it to a different volume (other than the one running El Capitan) will it leave El Capitan alone, allowing me to then have a dual boot iMac that can run either El Capitan or High Sierra?

I just don't want the upgrade to erase El Capitan, and I forget how these things work. I'm hoping it's as easy as this:

• Wiping the volume that Lion is currently on
• Having the updater ask me on which volume to put High Sierra
• Select the volume that Lion used to be on
• Ending up with High Sierra one on volume and El Capitan left untouched on its current volume

Is it as easy as that? Am I missing anything?