Hi! No, that would not be possible. For one, the A1278 is not even a Pro, and it has a removable battery with a different connector on the battery and the board than the unibody Pro machines. Also, as a non-Pro, the topcase/keyboard would not support illumination. Also, most Pros have an SD slot and a different display port than the A1278, so the ports on the board are not even going to fit the holes for the ports on an A1278 topcase. I see why you would ask the question, because on the surface they appear to be fairly similar, but under the hood they are not the same machine and you're going to run into lots of various subtle differences.
Wow, I looked it up, and it does in fact look like there was a 2009 A1278 Pro. I'm surprised, because I've dealt with at least a dozen A1278s, and they have all been non-Pros with a removable battery.
Honestly, I don't know for sure whether an i5 board would work in that machine, but I would suspect not because between models Apple tends to change screw locations, connector types, etc. I checked out the guides on ifixit.com, and offhand it appears the i5 board has a drastically different heatsync, so I'm not sure if that would affect how it fits in the case, etc. Beyond that I didn't see anything that appeared strikingly different, and they do both use the same DC-in, which is a good sign. You might want to post on ifixit.com's Answers page, and maybe someone there will know for sure, but like I said, if I had to bet, I'd say there's probably a couple issues that would make it incompatible.
Good luck, and if you find out let me know, because you've made me curious.
Just to follow up, I came across a liquid-damaged 13" Pro i5, and I can definitely now say that there are differences that would present issues. The connector on what is probably the iSight cable is smaller and completely different, which means you'd need a different cable, or a different screen assembly entirely (I'm not sure if it's simply a matter of swapping the cable out of the hinge, or if it's more difficult than that). Also, the heatsync is smaller and dramatically different looking. There seems to be a few other minor differences as well, such as that the metal piece that screws down and holds the video cable in place is a different shape, and the screw locations for it are probably also different as well. The i5 also has a media card slot that at least some of the A1278s (the non-Pro A1278 is the one I am looking at) definitely do not have, which means they holes for the ports would not line up.
So, again, I've confirmed there are many differences that would make using an A1278 body difficult, if not impossible, with a 13" i5 board.
Just a point of note: I've been referring to the i5 13" Pro as if it's something other than an A1278, but in fact it is still an A1278, just like the far older non-Pro unibody 13" MacBook. Kind of like the older white/black A1181 series, the A1278 model number stays with the machine even as several generations of improvements occur. I guess they will probably call it an A1278 as long as it retains the current 13" unibody form factor.
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if so, do you feel that it would be more likely that an i5 will fit?
Honestly, I don't know for sure whether an i5 board would work in that machine, but I would suspect not because between models Apple tends to change screw locations, connector types, etc. I checked out the guides on ifixit.com, and offhand it appears the i5 board has a drastically different heatsync, so I'm not sure if that would affect how it fits in the case, etc. Beyond that I didn't see anything that appeared strikingly different, and they do both use the same DC-in, which is a good sign. You might want to post on ifixit.com's Answers page, and maybe someone there will know for sure, but like I said, if I had to bet, I'd say there's probably a couple issues that would make it incompatible.
Good luck, and if you find out let me know, because you've made me curious.
John
So, again, I've confirmed there are many differences that would make using an A1278 body difficult, if not impossible, with a 13" i5 board.