I agree that the better idea is to buy a off the shelf camera.

However, from the Nokia service manual it clearly shows the camera is a single-chip device. It's a nice square chip package that fits into a socket on the motherboard. That said, I'm sure the chip manufacturer makes other chips with the same foot print and pinout that one could pop in and replace the default with. That said, this may also be the best module for this chip footprint, and/or with the lens format. (eg To go above 5mp you may need a different socket.)

Even if you could find a hardware compatible chip with a higher resolution and lens system, there's no guarantee that the software in the phone could handle the new chip. Most of the chips also come with a rather custom driver, and/or chip-specific settings profile. I work with embedded CCD devices as part of my job and can tell you even the same controller capable of using alternate CCD chips needs to be reconfigured for each CCD, even those put out by the same manufacturer.

So, is it possible? For an engineer at Nokia, with access to the source and the config files, probably. But for most anyone else, probably not. As I said, I work with this type of equipment every day, and even I wouldn't think of trying this on my device.