HOW TO UPGRADE TO YOSEMITE

That is correct. Yosemite is not longer available for new downloads. Only if you have previously purchased Yosemite, then it would be available via the Apple ID used before in the App Store -> Purchases tab.

You can go to El Capitan (free upgrade)

<https://support.apple.com/en-us/HT206886>

but be aware that Rosetta (runs older PowerPC based apps) is no longer part of OS X starting in Lion 10.7. So make sure all the apps you need are going to run.

You should also be able to upgrade to Lion or Mountain Lion ($20 US). You would go to the Apple only line store, purchases a redemption code that gets sent to your Apple ID associated email address. Once you have the redemption code, you can use Applications -> App Store -> Featured -> Quick Links -> Redeem to get the Lion or Mountain Lion you purchased.

If your Mac is compatible, you can also go to Sierra

Check to make sure your applications are compatible. PowerPC applications are no longer supported after 10.6.

Application Compatibility

Applications Compatibility (2)

El Capitan 10.11 Compatibility information

macOS Sierra 10.12: Compatible Apps

Also check to make sure there is a compatible driver for your printer.

Do a backup before installing, preferable 2 backups on 2 different drives.