One method to prevent an automatic conversion when installing High Sierra is to redirect the install onto an external drive first.
(Or a traditional HDD on larger desktop machines and laptops that were modified to have these drives in addition to SSDs.)
From the external drive this installation can then be cloned back to the main internal boot drive (preferably after the boot drive is erased). Once the clone is back on an SSD the filesystem will stay at HFSplus.
If the goal of the whole procedure is a "clean install" anyway then this is less of a detour as it may seem. It might be actually faster in this way, if the options involve a slow USB-stick you have to prepare for this with
createinstallmedia and then install from there vs. first installing onto a very fast external drive. [That same possible advantage may be desired when someone wants multiple machines to upgrade to High Sierra. See update2 below]
No external drives will currently be converted to APFS when installing macOS 10.13.
No-one really knows for how long this principle will hold true but currently this is the case for both kinds of drives: rotational magnetic and solid state drives.
Since this is met with so much opinion, others suggest:
Safest way to upgrade to macOS High Sierra without APFS
The way that I strongly suggest doing this is the safest and easiest. Safest because if something goes awry, the upgrade can be dropped without changing what you already have:
- Clone the boot drive to any spare external drive.
- Boot off the spare/clone.
- Upgrade the spare/clone. The updater will leave the file system alone—it won’t be converted to APFS.
- Boot off the now updated spare/clone. Check things out and verify that things are working to your satisfaction—and don’t rush this. Make sure everything is acting right and that everything you normally do is OK as well.
- Satisfied? Clone the now updated external to the original boot drive.
- Boot from the original boot drive (now updated).
By doing it this way, there is no change to the existing boot drive until you explicitly choose to clone back over it with the updated system.
Please note that a "clean install" is different from the in-place upgrade now often preferred by users: a clean install means that previous programs, settings and data should be erased (via formatting) before the new installation is cloned back and therefore have to be restored afterwards either manually or with Migration Assistant.
While a clone of a fresh install may be performed similar like an upgrade – just on top of whatever is on your boot drive – this method is not advisable. Formatting the internal boot drive before cloning back the new and fresh install ensures that the internal drive is in just the same "really fresh install" condition that is the primary goal of performing a "clean install".
Update2: Upgrading multiple machines in this way is not recommended by Apple anymore, since firmware updates are now rolled into the installer. Unless we figure out how the firmware updates are to be applied manually. Although this issue seems to be more pressing with APFS installations than with HFS+ as outlined in the question.
The clone route outlined above is currently only recommendable for upgrading a single machine (where a firmware upgrade will take place) or re-installing the operating system on machines that had the firmware updates coming with and in High Sierra already applied.