For some Mac users, macOS 10.15 Catalina is no longer a choice. That’s because the new 16-inch MacBook Pro and the 2019 Mac Pro that Apple released late last year ship with Catalina installed and can’t run any previous version of macOS.

But for most people, it’s time to consider an upgrade to Catalina. Most backup software now works with Catalina’s bifurcated drive approach that puts the system on a separate, read-only volume from your data and apps. We’ve all had several months to come to terms with the fact that old 32-bit apps won’t even launch in Catalina. And Apple has shipped several updates that bring Catalina to version 10.15.3, addressing most of the complaints users had with the initial release.

If you are ready to try Catalina but still want to use 10.14 Mojave, we have some advice for how to make that happen. This could be the case for someone who has purchased a new Mac that does support Mojave but came with Catalina installed, for someone who wants to test Catalina while still using Mojave, or for someone who wants to move on to Catalina but has a 32-bit app that they aren’t ready to say good-bye to.

Downgrade from Catalina to Mojave on Some New Macs

Apple has started installing Catalina on new Macs other than the 16-inch MacBook Pro and 2019 Mac Pro, but since these older Macs can still run Mojave, it’s possible—if a bit tricky—to downgrade them to Mojave.

System engineer Armin Briegel has worked out a way of downgrading new Macs to Mojave. First, you create a Mojave Installer USB drive. To use that drive to boot a Mac with a T2 security chip, you must allow external booting from the Security Utility on the Recovery partition. Once you’ve booted from your Mojave Installer drive, use Disk Utility to erase the entire internal drive. Then install Mojave.

Use Virtualization to Keep 32-bit Apps Running

For some people, what’s keeping them on Mojave is a single 32-bit app that will never be updated in an appropriate fashion. Quicken 2007 falls into this category, as does the ScanSnap Manager app for the ScanSnap S1300, S1500, and S1500M scanners. Sure, you can get a current version of Quicken, but it may not do precisely what you want, and Fujitsu would be happy to sell you a new ScanSnap scanner that does come with 64-bit software, but then you’ll have to figure out what to do with your old scanner.

So if you’re ready to upgrade to Catalina in general but need to maintain access to one or two apps, one solution is virtualization software: either Parallels Desktop or VMware Fusion. Both apps let you run nearly any operating system—including older versions of macOS like Mojave—in a virtual machine. In essence, they fool the guest operating system, whether it’s Mojave or Windows, into thinking it’s running normally on a computer, when it’s actually running in a virtual environment.

These apps cost about $80, and while there’s a bit of work in setting them up (the screenshot below shows the option for installing Mojave in Parallels Desktop during setup), once you have them configured, it’s easy to run older apps alongside newer ones with little or no performance hit. This approach is also perfect for a 16-inch MacBook Pro or Mac Pro that can’t run Mojave in any other way.

Install Catalina and Mojave on Separate APFS Volumes and Switch Boot

Finally, there’s one other option that lets you switch back and forth between Mojave and Catalina, assuming your Mac supports Mojave. You can create an APFS volume on your internal drive and install another version of macOS on that. It’s easy, and Apple provides full instructions. The only problem with this approach is that you’ll have to restart to switch operating systems, whereas both are available simultaneously with the virtualization solution.

First, make sure you have at least one current backup of your Mac, since it’s foolhardy to adjust your drive structure without one. Next, in Disk Utility, select your internal drive, choose Edit > Add APFS Volume, and click Add. We recommend naming the drive such that it will be clear what’s on it.

Then boot into macOS Recovery and install the desired version of macOS on your new volume. The keys you hold down to get into Recovery determine which version of macOS you’ll get:

• Command-R reinstalls the latest version of macOS that was installed on your Mac.
• Command-Option-R upgrades to the latest version of macOS that’s compatible with your Mac.
• Command-Shift-Option-R reinstalls the version of macOS that came with your Mac, or the closest version still available.

Choose Reinstall macOS from the macOS Utilities window and proceed from there.

Once the installation is complete, to switch from one version of macOS to another, open System Preferences > Startup Disk, choose the desired volume to boot from, and then click Restart. Or, press Option at startup and select the desired volume from the Startup Manager screen.

Needless to say, the decision about when and how to upgrade to Catalina isn’t a trivial one, so feel free to contact us to discuss your particular situation or to get help with any of the procedures that we’ve described in this article.

(Featured image by Apple)