[fouc]

You should downgrade it back to MacOS 10.x version range. I'm still on 10.13 myself, I don't feel any strong urges to upgrade to Mojave or Catalina. If I stopped using homebrew and started using macports or pkgsrc I'd probably be even less bothered by needing to upgrade. (homebrew is pretty shitty with older OSes)

I'm also on a 2015 MBP that's still going strong. It's snappy and I'm happy. I upgraded the SSD last year, and that made a dramatic difference as well. Upgraded to 1TB, write speed jumped from 282 MB/s to 1321 MB/s, read speed jumped from 964 MB/s to 1505 MB/s. (2015 is the last year that Macbook Pros had upgradeable SSDs)

[tistoon]

I would like to upgrade my disk while keeping the OS version (clone?). Could you please tell me how did you do?

[fouc]

I cloned to a USB drive, swapped the SSDs, and then booted up with the USB drive, and cloned back to the new SSD.

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You will need an NVMe SSD adapter similar to this https://www.amazon.com/Sintech-Adapter-Upgrade-2013-2015-Mac...

There's a whole thread writeup here: https://forums.macrumors.com/threads/upgrading-2013-2014-mac...

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You can also download old versions of MacOS installers. I forget the link to the easier scripts/installers, there's a High Sierra one at http://dosdude1.com/highsierra/

Personally I ended up using ./installinstallmacos.py from https://github.com/munki/macadmin-scripts to grab the catalog contents

and then temporarily blocked swcdn.apple.com in /etc/hosts,

and then served the /content/ directory so that swcdn.apple.com/content/ was pointed at 127.0.0.1/content (I used python here in the parent directory of /content/) > sudo python -m SimpleHTTPServer 80

and then I opened the High Sierra MacOS app store specific link to make it download & assemble the .dmg file properly

Open in Safari to ensure it launches App Store https://itunes.apple.com/us/app/macos-high-sierra/id12462847...

Can use the same approach to grab other versions of MacOS downloaders from apple too if you look up the app store links for it and then serve the catalog locally I guess.

[Cockbrand]

I did the same with my 2015 MacBook Pro, and I'm still amazed that a $5 adapter and an Intel SSD are super fast and don't cause any problems at all. The machine sleeps, wakes, hibernates without any issues whatsoever. Also, I sold the original, well used Apple 1TB SSD for more than I paid for the brand new 2TB Intel SSD.

[AnonHP]

Not GP, but as others have described here, you can clone your entire disk to an external one (using an application like SuperDuper! or Carbon Copy Cloner), upgrade the internal disk, and the restore from the external clone.

When it comes to hardware upgrades of the DIY kind, iFixit (https://www.ifixit.com/) is a great resource with detailed instructions, photos and even videos on how to do them for different devices. The site also sells tools and parts, but you don’t necessarily have to buy from there. For SSDs, OWC (Other World Computing, https://eshop.macsales.com/) has tested parts available for many older Macs.

[avtar]

I've used SuperDuper (both, the free and paid versions) to do this in the past.

https://shirtpocket.com/SuperDuper/