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I'm going to be upgrading my Macbook Pro (mid-2012 model) from its original OS X Lion 10.7 installation to a newer version of macOS. It looks like Apple will allow me to upgrade from Lion directly to OS X El Capitan 10.11 and macOS Sierra 10.12, and from there all the way to macOS Mojave 10.14.

I have heard that upgrading to a newer OS can cause performance issues, especially on older hardware. How can I find out about these issues, so I can determine the best version of macOS to upgrade to?

The biggest consideration is the software you run, not just in terms of compatibility with macOS generally, but also in terms of identifying if they're 32-bit or 64-bit apps (which will be a crucial consideration moving forward if macOS Catalina interests you, since it will be compatible with your MacBook Pro hardware).

As a starting point you can use the RoaringApps website to check software compatibility. You can search by specific application if you have key software you need to work with. You can also actually download a Roaring Apps app for macOS, but I've always found the Application Compatibility Table (see below) on their website to meet my needs.

My advice would be to check your software against the Application Compatibility Table for compatibility with El Capitan. If all good, then upgrade to that first and use the system for long enough to be satisfied everything is okay, and long enough to have a full working backup of your El Capitan setup.

Once you've done that, check the Application Compatibility Table again for compatibility with macOS Sierra etc before upgrading again.

To identify which applications you have installed are still 32-bit, you can follow one of the answers at: How can I find out which Mac apps are 32-bit?

What should I look out for before I upgrade, and should I consider not upgrading to the newest version of macOS so I can keep certain applications running the same?

Use the following reasoning to answer the above questions. At the simplest level, this is what you need to take care of:

1. 
   

   Prepare a list of all the 3rd party apps that you use. Make sure that it includes, at the least, the apps that you are highly dependant on for your day to day personal/professional usage. You'll need to make sure if they are supported and run fine on macOS Mojave.

   
   
   

   Mac OS X Lion and macOS Mojave are 7 years apart. With the pace of technological innovation, quite a few apps have broken/become unsupported in the newer releases of macOS.

   
   



2. Prepare a list of all the 1st party (Apple) apps as well. Especially list down the apps that you are highly dependant on for personal/professional usage. Apple too has deprecated/removed some of its apps. You need to make sure that upgrading to macOS Mojave doesn't leave you hanging without some apps that you are dependant on.



3. 
   

   Apple announced a while ago that macOS will drop support for 32-bit only apps. While this was expected to happen with macOS Mojave, 32-bit apps still run, albeit with an error message that the app may stop working in the future version of macOS. (Such apps still work with the current and most likely the last public release of macOS Mojave, i.e. 10.14.6. This will change with macOS Catalina, the upgrade expected within a couple of months).

   
   
   

   Check if any of the apps that you use (3rd party) that are still 32-bit only.

   
   

Check the release notes for or get in touch with the developers of the 3rd party apps you are concerned about to check their compatibility status with macOS Mojave. This will better prepare you with your upgrade plan.

Would keeping a separate partition of macOS Lion be an alternative?

Yes. However, it would require some work with creating partitions and installing Mac OS X Lion and macOS Mojave side-by-side. This could be some work for a casual user.

I'd recommend you to get an external USB hard drive and install macOS Mojave on it. You can boot off of the external drive (restart MacBook and hold Option key to select boot drive), while leaving your current installation of Mac OS X Lion intact. Work with this setup for a few days until you are convinced to fully commit to macOS Mojave.

Do note that when running off an external hard drive, macOS Mojave may feel slow merely due to the fact that the OS is running off of an interface with slower speed. Don't let that make you assume that macOS Mojave is slow.

This is the best and the easiest strategy you can use to test things out and transition seamlessly.

The problem in upgrading is largely one of compatibility with your existing application software. If you are running software designed for Lion (released in 2010), then jumping up to El Capitan (2015) or later may cause problems, as the software you've installed may be expecting things that aren't there or don't work in the same way. Make sure your apps and other software is compatible with the newer OSes, and you should be fine.

While your hardware still falls into the range of newer OSes, they usually are geared towards faster, more capacious hardware, and so work better on systems with SSDs and lots of RAM.

However, I had a 2012 MBP, and it worked perfectly fine on every OS I upgraded it to (albeit with an SSD in place of the original hard drive and 16 Gb of RAM). I also made sure that my software was broadly 'contemporary' with the OS.

You don’t have to do any pre testing if you get an external USB drive and install any new OS you choose. As long as it’s newer than the one you run, you can then boot to the external, run migration assistant to copy over your apps and data and run any tests you like.

If it is faster, do the upgrade on your main disk and repeat. If not, you can spend whatever time you want sorting out the performance or skip that version and erase the external.

Why worry about general benchmarks when you can test the things you really care about and then upgrade when you are prepared and have data to back up your move?