SOLVED: macOS UPGRADE ISSUE AFTER INSTALLING 1TB INTERNAL SDD ON 2013 LATE 27" iMac
Okay, it took a lot of trial and error. But I finally figured out after my iMac reset to the original OS as that's the only thing that allow you to do after you install the SSD unless you made a clone copy of your hard drive or were able to before you installed the new SSD.
The solution was to do a internet rescue for the original hard drive then go to this site: http://dosdude1.com and download each OS you want to upgrade your new SSD to. Download each “patcher" app and read instructions as the app will download full version from Apple. Upgrade by order. Meaning use the oldest OS then upgrade again and again until you have Catalina. Then Catalina will do a final update from Apple through your Settings > Software Updates on your MAC. This works perfectly and makes the new Cruical SSD accept all OS upgrades. Apple obviously has put in the cose it doesn't like people doing self repairs and tries to block you from updating OS BUT.THIS SOLUTION SOLVES THAT!
PLEASE NOTE: you are using the Patcher from the Above website for each OS you want to download. You're only using it for the download from Apple so you can get a full copy easily. When it downloads put the new full version of the OS on your desktop. Do not patch it as the software is originally used to do. I repeat do not patch through the Patcher only use it to download the full versions. After you have the full version on your desktop you can double-click on it and it'll upgrade your OS just like any other time. You do not have to put it on a USB or external disk to boot up on. Just click on it and let it upgrade your machine. Just remember don't patch the OS you want to install each OS in order right from your desktop until you get to Catalina. Then update Catalina from your settings app.
http://dosdude1.com DOSDUDE1 .