You may not be able to open version 12 projects in version 14, so just don't uninstall version 12 and you will be able to open version 12 projects. You can have many different versions installed.
The "upgrade" is just a marketing term, as you can usually buy the full version for less than the "upgrade" version (which is really a full version) from an authorized Adobe retailer or online seller
So it's possible I won't be able to transfer the projects I now have in premiere 12 to premiere 14?
You may or may not.
Download the trial and you can see if you can.
I would not recommend doing so. It can lead to file corruption.
But you can keep version 12 and 14 on your computer at the same time -- and finish your version 12 project in version 12, starting your new projects in version 14.