In my previous article on my High Sierra upgrade I mentioned you could choose to NOT upgrade your Mac to the APFS file format during the install. If you have a lot of legacy apps, it might be better to stay on the HFS+ file format for a few months. I mentioned during the install you will see a window like this:
As you can see from this image, I chose to upgrade my file system to APFS during the install. However, if you have decided to remain on the HFS+ format, but would like to upgrade your file system to the APFS format at a later time then please read this article which explains how to boot into Recovery Mode and use the Disk Utility app to do just that. Remember, you pretty much need to be doing this on a Mac with a SSD.