How can you prompt a newer OS to restart from an older OS?

[Agincourt]

I presently have 10.15 on my machine and I want to install OS 10.13 on its own partition.

Problem is that anytime I get the older version for download, the newer system rejects it and won't even allow me the option to run the program. I likewise cannot restart the computer and press 'option' to allow me to upload anything except what's already present. How is it that you can download a new OS if your drive's blank but you cannot upload an older version on the same machine? Almost every time I do a partition for this purpose I always end up with only one OS despite the partition?

How can I prompt the computer to seek out an OS from the internet and ignore the OS already present?

 

[DeltaMac]

If you already have a system installed (and you are booted to that system) - when you download a different system, your Mac will warn that the system is too new (or too old).
So, you can cancel, or just quit the installer that you downloaded. It will remain in the Applications folder. (If you use a method to download, such as MDS, then you can choose some other folder to save the downloaded installer app.) Use the terminal to create a bootable installer on an external drive. Boot to that installer. Make sure that you second partition is ready for the install, checking in Disk Utility that there actually is a destination partition for that install, and it is the size that you want to use for that additional boot system.

 

[Grumpus]

See here for instructions on creating a USB installer, in particular the command for High Sierra. At the end of the command, MyVolume is the name of the USB you're installing to and you may need to change that name.