Yosemite should be no different than Lion, Mountain Lion, Mavericks when upgrading an internal drive.
You could (assuming Yosemite is your current OS):
Back up your current system. Then create a USB thumb drive using the OS X Recovery Disk Assistant utility which puts the Recovery HD on a thumb drive so you can use OS X Utilities. Install your 512GB SSD, boot from the thumb drive, use Disk Utility to format the SSD then reinstall Yosemite to the SSD. You will need an Internet connection because this will download Yosemite from Apple then install it. During Setup you could migrate your user account(s), apps and data.
Back up your current system. Then create a USB thumb drive (8GB) installer for Yosemite. Install your 512GB SSD. Restart using the USB thumb drive. Then use Disk Utility to format your 512 and install Yosemite. During Setup you could migrate your user account(s), apps and data.
Back up your current system. Then clone the 120GB SSD to the 512GB SSD or USB drive by restarting into Recovery Mode (Command+R). Use Disk Utility - Restore to clone the 120 to either your 512 using an external USB drive enclosure or to an external USB drive. If you clone using the external enclosure, then you would simply install the 512 and your good. If you use a USB external drive then you would install your 512, boot to Recovery Mode on the USB external drive then use Disk Utility to format and Restore from the external drive to the 512.