Usually, the only upgrades you can make to a laptop are additional memory, or swapping out the hard drive for a solid state drive (SSD). I don't know of ANY laptop brands that allow you to upgrade a CPU. Even if you could, I'd save your money.
If you want to make an old laptop feel snappier, add some RAM, or pick up a low profile SSD, like the Samsung 850 EVO.
That said, the i7 is a bit faster, and is a quad core vs a dual-core, which will make a difference in the newest games, or if you actually do CPU-intensive tasks (encoding/decoding video/audio, casting, etc...)