New OS versions not playing nice with ancient backups, etc.
I should of asked, what OS are you currently using, and what OS will you be using on the SSD?
I ran into an issue with the OS when swapping the HDD to a SSD on a Mid 2011 iMac.
My friend found an iMac and a Power Mac G4 in a dumpster at his work. He gave me the Power Mac to add to my collection, but wanted the iMac. He couldn't get it to work, so he ended up giving it to me as well.
He had no idea of the specs because it wouldn't boot. I discovered that the boot drive was corrupted and the GPU failed. I took the iMac totally apart to remove the GPU and successfully did the oven bake to it to fix it.
As I put it back together I swapped the HDD for a SSD with the iFixit temp sensor SATA cable adapter.
I booted fine, but with no OS on the drive. I used Target Disk Mode and CCC to clone the boot drive with High Sierra from a Late 2011 MBP to the new SSD in the iMac.
Everything worked great. The OS booted, and the GPU was working well.
The first thing I did when I first got it booted was look at the specs. This dumpster iMac ended up being a max upgraded BTO iMac from Apple. It had the 3.4GHz i7, the 2GB AMD Radeon HD 6970M, and 16GB of RAM.
The display looked as beautiful as my Late 2012 iMac.
Now, for the OS problem.....
I decided to give it back to my friend that gave it to me for the cost for the SSD. But before I did, I attempted to reinstall High Sierra with a fresh install for him.
The problem was that I was unable to install High Sierra onto the SSD. On the installer app, it showed the SDD as being greyed-out.
It took me a few hours to figure it out, but it ended up being that the original HDD only has Yosemite installed on it. I was able to install up to Sierra on the SSD, but not High Sierra. The reason why is that there was firmware that needed to be installed for High Sierra to be installed on an SSD.
The new issue was that I needed to install High Sierra on an Apple HDD to install the firmware. I didn't have one except the one that barely worked. Also, it needed to be installed internally to install the firmware.
So, I swapped the new SSD for the barely functioning HDD. I was able to install the High Sierra on the HDD and with it, the firmware installed automatically.
After that, I again swapped the HDD for the new SSD, and I was able to do a fresh install of High Sierra. I am unsure if your iMac would have the same issue, but it only takes a few minutes to install High Sierra if you did not already, this way you know for sure that you will not have this issue.
TLDR: Make sure you install High Sierra onto your internal HDD before making the swap for a SSD.
I'm normally a "just get the new model" kind of guy, but I don't do nearly enough with this iMac to justify such an expense.
I am not knocking anyone that wants to buy a new computer, or have the latest and greatest Mac. If they can afford it, it is their money, and they can do what they want with it.
I just think that on the MR forum, some people are so quick to recommend buying a new(er) Mac without considering options to keep an older Mac around.
If one is mostly happy with their older Mac, but would like something simple improved, like disk speed, why spend $2000 on a new Mac when a $100 upgrade would accomplish what the user is asking for.