If the HDD only model uses the "top" bay (bottom when turned upside down ...), then it's a piece of cake, you don't have to do 2/3rds of what's outlined in that iFixIt DIY, assuming you don't want to pull your HDD and want to +add+ an SSD.
I had an i5 HDD model '12 Mini, swapped out the HDD for an SSD, then changed to a QC i7 '12 Mini, and I was able to remove the i5 SSD, reinstall the OEM HDD, and remove and replace the "bottom" HDD in the i7 machine in < 30 minutes. It's really very simple, just a couple of steps you have to exercise a little additional caution as you're dealing with very small connectors.
Once you're running off your speedy external SSD, that pokey 5400 internal makes a great place to dump your music library, some books, old photos, even a Time Machine or SuperDuper! backup.
Yep, that's a great option. I have a server spec QC i7 '12 Mini, I pulled the bottom drive, swapped in my MX100 SSD, left the top 1TB HDD in place, it's where all my VM backups, media, client archives, and some other "offline" files reside.
In fact, I need to migrate more "slow files" to the HDD!