Replacing HDD with SSD; what about non-APFS/APFS?
No, you don't need to convert the HDD. I recently did exactly what you're doing (though I installed the new SSD myself, which meant not having to worry about erasing the HDD) and didn't have to do any conversion.
But, just to be on the safe side, it would be smart to not only make sure you have a Time Machine backup but also clone your HDD to a different drive than the one that has your Time Machine backup. That way if, for whatever reason, you have a problem with restoring from the Time Machine backup, you have a fall-back plan. Better safe than sorry. You can clone the internal HDD to a second drive using Apple's Disk Utility or something like Carbon Copy Cloner. Instructions are readily available.
The other thing I'd suggest is that instead of simply restoring everything from your Time Machine backup to the new SSD you do a clean installation of macOS onto the new drive and then just restore your files and data using Migration Assistant. That way you get the benefits of a fresh OS installation along with the benefits of your new SSD.
No - I wouldn’t do anything for the HFS -> APFS transition if you are on an SSD. This has been vetted for months across many architectures and as long as you have a backup of the data, proceed.
I wouldn’t make any upgrade without a backup, so YMMV if you go without recent backups - any change at that point might not be worth the risk - even letting the system run is a risk if you have no backups and would want any of your time or data back when it inevitably gets corrupted or fails.
My recommendation would be to move all the data to the SSD using an external bus connection and test that restarting to it works before you open the hardware and do any swap. Lots of people put the SSD in and then try to worry if the part is tested or the cables are correct. Do all the testing and data migration before you put in the drive is my main advice.
- Install the OS you want on the external SSD.
- Run Migration Assistant to move all the apps and data to the SSD (when booted off the SSD).
- Test things with the SSD running everything. Including set up backup of the new SSD to whatever destination you want for Time Machine.
- Run for a week - letting the internal HDD just be there, not even mounted if you prefer.
- Do the swap SSD for HDD.