First lets work with some facts, here's the IFIXIT guide to replace the systems logic board: MacBook Pro 13" Unibody Mid 2012 Logic Board Replacement. If you jump down to Step 27 you can see the CPU is firmly attached to the logic board (soldered) so swapping it out is just not possible without swapping out the entire logic board.
As to what boards fit, its best to stay within the model series. In your case this would be the MacBookPro9,2 Series. As you can see this series had only two systems a i5 (2.5 GHz) and a i7 (2.9 GHz).
Frankly, I think the amount of improvement is not enough to warrant swapping it out. I can think better things like upgrading your current HD to a SSHD hybrid or even a SSD drive either would be a better investment.
But before you do anything use your system as is it might meet your needs as is!
You may want to run disk utility to make sure the permissions and drive are in good shape, then do some system house cleaning deleting the old cache and log files. Lastly consider defrag'ing the drive as over time they do get fragmented. You should try to leave 1/4 to 1/3 of the drive free for the OS to do its thing.