Upgrading from Mojave 10.14 to Catalina 10.15 on iMac 2019? Possible? How?

An iMac (Retina 5K, 27-inch, 2019) can run anything from macOS 10.14.4 (Mojave) to the latest release of macOS 15 (Sequoia). Sequoia is still one of the "most recent three", and will be for about another year, until macOS 28 comes out.

None of the other operating systems that your Mac can run are among the "most recent three", so unless you need to stay on Mojave (to run 32-bit applications), you'd probably want to upgrade to Sequoia. (I would suggest making a bootable backup of your current startup disk first, using Carbon Copy Cloner or SuperDuper!)

How to download and install macOS - Apple Support

Catalina (10.15) was released in 2019, so there's no reason why you can't run it on your machine. Any reason why you've waiting 7 years to run this update?

Are you saying you've tried to update and it failed? or are you asking how?

You should be able to use the build-in Software Update (System Settings -> General), but if that's no longer showing Catalina updates then you can download the relevant installed from https://apps.apple.com/us/story/id1784326336

> ...in order to use ChatGPT AI on my machine, apparently I need to have Catalina installed

Not so...

In order to run ChatGPT, you need to have at least Catalina installed.

The App Store/Software Update will always suggest the most recent OS version available for your system - in this case Sequoia. Since Sequoia is later than Catalina you'll be able to run ChatGPT just fine.

You can use the link in my earlier post to get Catalina if that's your preference, otherwise trust what Software Update is telling you.

Yorsmoo wrote:

I believe I couldn't upgrade the Photoshop app to a newer OS without paying Adobe a subscription so I've left it all these years. That said, Photoshop constantly crashes on this iMac so I have to use it on my older 2012 iMac which still works great with it.

One-time-purchase versions of Photoshop aren't supported under Catalina and later.

Catalina drops support for 32-bit applications. I believe that all one-time-purchase versions of Photoshop have 32-bit installation and DRM activation code. So even if the main application was 64-bit, and didn't break due to some other macOS change, you wouldn't be able to successfully do a clean reinstall under Catalina and later.

If you don't need Photoshop, per se, there are non-subscription alternatives like Affinity Studio. If you must have Photoshop, Adobe pretty much has you where they want you – and these days, the $10/month Photography Plan isn't available to new subscribers. You'd be looking at a $20/month plan.