how safe is upgrading high sierra to yosemite?

As previous posting indicated - High Sierra was released in 2017 and Yosemite in 2014.

That or the user mis-typed Upgrade but really meant Downgrade ?

Either way and depending on the exact vintage of this MBP 2012 - it may qualify to run Catalina macOS 10.15 released 2019 or Mojave macOS 10.14 released in 2018

macOS Catalina 10.15 hardware requirements

macOS Mojave 10.14 hardware requirements

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

macOS Mojave - Technical Specifications

General Requirements

• OS X 10.8 or later
• 2GB of memory
• 12.5GB of available storage (OS X El Capitan 10.11.5 or later)*
• Some features require an Apple ID; terms apply.
• Some features require a compatible Internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Mojave:

• MacBook (Early 2015 or newer)
• MacBook Air (Mid 2012 or newer)
• MacBook Pro (Mid 2012 or newer)
• Mac mini (Late 2012 or newer)
• iMac (Late 2012 or newer)
• iMac Pro (2017)
• Mac Pro (Late 2013; Mid 2010 and Mid 2012 models with recommended Metal-capable graphics cards)

guanche wrote:

I wonder if this mid-2012 MBP would support Mojave...so any 64-bit OS versions are already discarded I guess.

from my previous answer…

FWIW, a 2012 MacBook Pro can be upgraded to macOS Mojave or Catalina. Catalina is 64bit only.

FWIW, a 2012 MacBook Pro can be upgraded to macOS Mojave or Catalina. Catalina is 64bit only.

How to get old versions of macOS - Apple Support

Yosemite would be a downgrade.

guanche wrote:

Thank you so much. An official website ask to update to Yosemite...lol

You're welcome.

Welcome.