I have done just this very swap. I started with
- a stock Macmini4,1 (Mid 2010 Mac Mini with P8800 2.66GHz Core2Duo, 320GB HD, SuperDrive) running 10.6 (for Rosetta) and 10.8 on separate partitions
- a stock Macmini5,2 (Mid 2011 Mac Mini with i7-2620M 2.7GHz i7, 750GB) running 10.9
Following the iFixit disassembly guide, in the process of swapping motherboards, I removed the optical drive tray and made the cuts as described in the thread. Two of the cuts aren't as necessary, as they can be shaved off a little and still fit with the 2011 motherboard. I also switched the HDs and swapped the SuperDrive for a Blu-Ray rewritable drive. However, I had later found out that the drive would not accept discs in the Mac Mini due to the Manufacturing Diagnostic pin in the optical drive power connector, and needed to be removed. While disassembling it again, instead of removing it, I just bent it in so that there would be no contact with the optical drive.
After this process, I ended with:
- a Mac Mini 2011 with i7-2620M 2.7GHz (and its Radeon HD 6630M), 750GB, UJ-267 Blu-Ray rewriter running 10.9
- a Mac Mini 2010 with P8800 2.66GHz (and its Geforce 320M), 320GB, no optical drive running 10.6 and upgraded 10.8 to 10.9 and sold it