Hello,
There's only one socket for the i7 2630QM: FCPGA988.
So you don't have to worry, if you find one, it will necessarily be compatible with your laptop. When I told you that I don't know these machines very well, there are things I'll be saying which are wild guess but I'm not surprised that you don't find high end L420 with i7 processors. Those machines are entry level Thinkpads and entry level often means low budget too, I think most of the people who buy this type of machine watches his wallet at first and then watches what are the specifications of the machine, in a second time. I would not be surprised that an high end L420 is the same price than an i3 or an i5 T420 for example. So I'm guessing it's easier to find low specs L420 than high specs ones.
But fortunately, its possible to change the components inside the laptop. So you can have the same level of performances than a T420 after replacing or adding components. You'll only end with a laptop which is not of the same build quality, more bulky and less resistant. But if it's for home usage, that's not really a problem. As far as I'm concerned, I don't like these machines because of their looks (speaker under the screen, buttons at the left and at the right of the keyboard, width of the laptop, internal design and so on), but it's just my opinion, of course.
About the fan, I don't know if there are different models across the various machines. I think so, because the CPU don't have the same TDP, I doubt that Lenovo put the same best fan system in i5 machines and in i7 ones (particularly the ones with the Quad Core CPUs). Someone of the forum will certainly be able to answer better than me you about that.
One thing too: if you change your 2.5 GHz Dual Core CPU by a 2.0 GHz Quad Core one, there are some applications which will run slightly slower. The use of the Quad Core is really interesting if you launch a lot of applications at the same time (or if the applications are developed to use all of the cores). On my T420 (which is originally a T420i), I had an i3 2310M. I replace it with an i7 2640M (the main differences are the frequency – 2.10 GHz for the i3, 2.8 GHz for the i7 – and the L2 cache – 3 MB for the first, 4 MB for the second). Well, for basic tasks, the difference is barely noticeable. It's more visible in game because the IGP has a better frequency, but that's all. I can also compare with my T420s, in which I have an i5-2520M. Same thing, there's not much differences for a day to day usage. I changed my CPU on the T420 because I could have one for 15 €, and that was a real bargain at the time. On the other hand, I bought the machine as dead but it wasn't – and it was even in pretty good shape and complete but without RAM and HDD/SSD, I paid it 50 bucks and I wanted to have and "high end" (or sort of) PC without spending too much. In fact, I paid 50 € for the laptop, 15 € for the CPU, 60 € for the screen, 4 € for the touchpad rubber, the rest, I already had. So I paid a total of ca. 130 € for a laptop I would have paid 250 or 300 € with the same specs but fully operational. One of the best bargains speaking of my Thinkpads.
The 2630QM, I doubt you'll find it for 15 €, it will be more likely around 60-80 €, so you must be sure it has a bit of interest for what you plan to do whith your laptop.
https://cpu.userbenchmark.com/Compare/I ... /2729vsm29: performance difference is not enormous.
W.
said, you can put an i7-2630QM on your laptop. I'm wondering, as I don't know well these series, but I'm pretty sure you have an i5-2520M or i5-2540M, with a 35 W TDP: will the native cooling system be sufficient for the i7-2630QM (45 W TDP)? If not, you could under heavy load experience throttling, a lot of noise and performance loss. If there's only one cooling system for these machines, use some good thermal paste and install TPFanControl for optimization. 
). Again at work, so,I have more time!