If you could find a cheap 3.2 (good luck), the swap is not difficult. If your 3.0 is in good shape, you could get some of the money back for it. I personally would keep the 3.0 ECU and harness (Mark B., this works doesn't it?).
Before going down this road, I would seriously consider spending the money on other modifications. Displacement makes everything better, but if you just need a few more ponies, there are easier ways to get there.
If your motor is really screwed up (fatigued block or badly threw a rod), looking for a 3.2 is a good plan, but otherwise, I'm not sure it makes sense. I pulled my 3.0 and spent a fair amount of money going through it, so I first looked around for a 3.2 block, but could not find one. In the end, I'm not sure it made any difference.
I was curious if it was possible to convert a 3.0 to a 3.2, not swap. I was really just seeing if it was possible, not considering it for myself. However, I thought both engines were built off the same block.
You can't upgrade a 3.0L to a 3.2L. The reason being that the 3.0L uses Iron Cylinder Liners which aren't nearly as strong as the FRM (Fiber reinforced Metal) Cylinder Liners that the 3.2L uses. With the stronger Cylinder Liners the 3.2L can have a larger bore and not have any problems.
I think if you bored out a 3.0L to 3.2L's the walls between each cylinder become so thin that they can't hold up to extended use.
Also, Its my understanding that the new 3.2L have lighter FRM pistons as opposed to the Aluminum pistons used in the 3.0L. (I'm not sure on this one)
If you want to increase the displacement on your engine, the JUN High Compression Pistons require a 0.5mm Overbore, which from what I understand is about as much of an overbore as you can do without causing any significant problems. It should increase your displacement from 2997cc's to something like 3050cc's or something like that..
It is correct that you cannot "upgrade" a 3.0 to be the same as a 3.2 because of the nature of the cylinder liners. They have to be cast as FRM from the start.
The basic block is the same, it's just different cylinders (larger bore and FRM instead of cast iron liners). A bunch of other little things changed on the 3.2 design as well... Valve sizes and design, pin sizes, crankshaft diameter, head gasket, piston heads are coated to work with aluminum cylinder walls, bigger injectors, etc. But the pistons are not FRM, they are still "regular" aluminum.
Lud and Edo are correct in that you can't update the 3.0 block to the 3.2 specs. You probably could get a custom crank and bore out the block to get close, possibly even larger (don't some race motors displace 3.5L?), but it would be expensive. You could increase the bore on the 3.0 to the 3.2 size, but you would have to get custom sleeves made from a very stiff and durable material. Nikasil (sp?) or something similar. At this point, you are talking big $.
Thinking out loud --> Is the crank the same on both motors? If the 3.2 crank has a longer stroke, you might be able to drop it in and get some of the displacement. If you do that, however, be sure to check internal clearences very closely. You might have to machine the block a bit to make it work.
I'm sure a call to Comptech would sort out what can and can't be done. They have built enough NSX motors to know what, if any, parts you can frankenstien.
David is correct, I have done this swap before (and in fact am doing it again next week) and did use the 3.0L intake, plumbing, and ecu / harness. David is also right (imo) there are alot cheaper ways to get 20 hp.
Using the 3.0L intake etc. resolved all the issues with FBW, emmissions, etc. I also used larger (Honda) injectors as both swaps are (will be) over-blown. <eg>