Head-to-head tests show that models as old as the iPhone 6S run iOS 13 at least as fast as iOS 12, and some devices see a performance boost.

The same is true for battery life: testing indicates Apple’s latest operating system puts no additional strain on the battery of older models.

Be warned: anyone who upgrades their iPhone or iPad to iOS 12 no longer has the option to downgrade. It’s not possible to go back to any version of iOS 11. Not that there’s much reason to want to.

This isn’t an unusual move. With today’s release of iOS 12.0.1, going to iOS 11 would be two jumps back, and it’s very rare for Apple to allow that.

Some informal testing reveals that if you’re still rocking an old iPhone, you should probably upgrade to iOS 9.2.1.

iApple Bytes put three different models of older iPhones — the 4s, 5 and 5s — up against each other in a variety of tests designed to see which of them was faster starting up, using apps and running Siri, Apple’s digital assistant. Half were running the brand-new iOS 9.2.1, which arrived yesterday, and half were still on iOS 8.4.1, which has been around since August 2015.

You can see their results below in iApple Bytes’ series of videos.

When you back up your iPhone before upgrading to iOS 9, you have a couple of choices. You can use iCloud or iTunes.

For your best, most comprehensive backup, connect your iPhone (or iPad) to iTunes. Seriously, iTunes backs up almost everything — and it’s your best bet for downloading iOS 9 when the upgrade becomes available today.

Here’s how to do it.