The Mid 2012 cMBP is a good solid computer with a beautiful screen, even if not retina. In comparison to the latest and best Apple has to offer it holds up well, and surpasses all in ease and ability to upgrade components.
I can concur that substituting a SSD will not void your Apple Care warranty. I took this exact same model to an Apple store when it developed serious problems, with among other things the motherboard needing to be replaced. Apple did so free of charge under my extended Apple Care coverage (being close to the three-year mark). Only noting the non-standard SSD I had installed, so aware of it, but otherwise no issue.
Do note that while you'll thus bypass Apple's outrageous asking price for a SSD, my understanding that their new configuration of preinstalled SSD's will run faster than that you can install in a cMBP. Technology marches on.
Apple will happily preinstall and charge you handsomely for their SSD, however again my understanding that a SSD compatible with a cMBP can never match the performance of that within a rMBP. Thus certainly from a cost standpoint, and likely most other considerations, you are better of installing your own SSD in a cMBP.
As also mentioned by another, you will require two simple tools (the small screwdriver and Torx) to install a SSD on your own. A simple procedure made all the simpler if you read up on the install procedure in advance, so understanding the nuances.
Do not overlook that part of that is formatting the new SSD drive (again, simple), and that you'll be beginning with a clean slate. So give some thought to how you'll be transferring or installing the OS.
Perhaps consider reverting to the Mountain Lion (10.8) OS which came standard in 2012. Others will argue that the latest OS from Apple is preferable. Your cMBP can run that, with the latest OS (El Capitan, or soon Sierra) having more bells and whistles; also able to run some newer software requiring it, if likely not some older software or games no longer supported. But aside from that the case can be made the last of the cats, or the previous Snow Leopard, remain Apple's best configured and solid OS's. Even though now supplied with the new OS, the hardware of the cMPB will support revision to the older OS, should one desire (and then you've some homework to do in figuring that out).
Back to the SSD. I did that upgrade on day one and never looked back. Undoubtedly the single best upgrade you can make. Highly recommended.