Converting my SSD from Journaled HFS+ to APFS

I can't remember having had troubles with updates on HFS formatted drives. Usually I just download combo updates to have them ready in case of need without continuosly downloading stuff, but still, nothing I remember having met on H.Sierra or Mojave.

Thanks for the info on external drives, Dan. This is extremely helpful.

Just to clarify, I actually have Mojave installed on my current SSD. I downloaded it last fall on my previous hard drive and transferred over to my new SSD with SuperDuper this spring.

I was trying to reinstall Mojave in an attempt to reformat my SSD in APFS. I want to make sure I can not only encrypt my SSD but also update my SSD in the future when Apple releases additional MacOS packages.

Do you still recommend downloading the MacOS installer drive onto a USB, Dan? If so, what's the next step?

Well that's the root issue then! Using a cloning App! find your SuperDuper and any other cloning app and trash them! Since 2012 I've not used any 3rd party cloning apps as they only create issues later on like what you've encountered here.

I don't recommend upgrading past Sierra on SATA based systems (HDD or SSD) High Sierra and Mojave will upgrade the file system from HFS+ to APFS during the install process. High Sierra had a lot of issues I just don't install it at all! The probe here is the way APFS works on older SATA systems as throughput issues with it limited queues over what PCIe/NVMe drives have.

OK, You still want to jump in the deep end of the pool ;-}

Make a Sierra installer USB as well as a released Mojave installer following this guide How to create a bootable macOS Mojave installer drive

After backing up your drive (TimeMachine) reboot under the Sierra drive so you can reformat the drive and install it onto your drive. Install the updates as required (IMPORTANT: Setup a new user account different than what you currently using for the installs). Then if you must run the Mojave OS installer in the same manner booting up under the installer USB and installing it. and its needed updates. At the end of the OS install you'll be asked if you have a a backup to restore this is when you want to plug in your backup drive so the Migration Assistant can restore your user accounts, apps & data.

Doesn't look like Sierra is available in the App Store anymore. My list of purchased applications in the App Store was reset when I switched my hard drive, so I can't assess it that way, either.

I was able to download the High Sierra installer to a USB, however. When I tried to install it through Disk Utility, though, I received an error message telling me I couldn't install an earlier version of Mac OS. I guess this makes sense: some of the support forums on Apple's website indicate that you can't download older OS versions if you have one of the newer ones already installed. Got an error message (still, the APFS problem) when I tried the bootable USB route with Mojave, too.

"Convert to APFS" is greyed out in Disk Utility for me. Should I just re-partition my volume as APFS instead? (If so, should I use APFS or APFS (encrypted)?) OR, do you think I should just hold off on this until Apple releases macOS 10.15 Catalina in the fall? Is it possible that that update might convert my SSD to APFS instead?

Your hitting the shoe cobblers dilemma! The cobbler can't fix your shoe with you wearing them! Thats why you need to create the bootable OS installer so the drive you are trying to fix is not being accessed.