I was able to accomplish this. Can't say that it was easy or obvious though...and my situation was a bit more complicated.
SHORT VERSION:
I had copy my iTunes Library.xml from the computer that has the upgraded library to the other computer, then import it. This also requires that the upgraded files are already in the same relative location - in my case an external drive moved from one computer to another.
LONG VERSION:
My original matched library was on my Mini. I wanted to end up with an upgraded 256mbps library. Instead of deleting and downloading anything on my Mini, I figured I would enable Match on my MBA and have it download everything - point my iTunes Media folder to an external drive and once complete, just copy the upgraded songs over to my Mini.
Downloading from iCloud took about 5 days to complete (~20K songs). Then I ran into the real problems. First off, I realized (too late) that while the folder structure would be the same - there would be some difference in file names because in my original library, most songs were mp3's whereas the upgraded library is all .m4a (AAC). If I copied them via Finder, the folders would have been properly replaced, but iTunes wouldn't have recognized most of the songs since it would be looking for the mp3 files. Sooooo....then I figured that I would copy the iTunes library files (.itl,.xml, artwork folder, etc.) from the MBA to the Mini, attach the external drive open iTunes and fire up iTunes with the Option button pressed to allow me to point to my .itl file copied over from the MBA. I have done this before when moving iTunes libraries without issue but no dice this time. With Option key pressed, iTunes opened, allowed me to point to the copied over .itl file....but then it was beach ball time - iTunes would never open, wasn't responding and I had to kill it. Tried this 3-4 times with the same result.
Decided that I would take a look at the "iTunes Library.xml" file copied from the MBA. I expected to see that it was pointing to my external drive as the path for all of my songs since that is how it was set on the MBA and the files were obviously on the external drive (don't even have enough room on my Air for my music library). Strangely, for each and every song, it was pointing to the default location: localhost/Users/<username>/Music/iTunes/iTunes Media/Music/. This made (and still makes) no sense to me....since again this is the Libary files my Air was using and everything was fine there. Not only does my MBA not have enough space for my library, but that path doesn't even exist there...nor is there an alias there pointing to the external drive. I am still puzzled over this.
I then tried to import the libary (File->Library->Import Playlist) and chose the Itunes Library.xml file. I decided to *uncheck* "Copy Files to iTunes Media Library when adding to library" - just in case this didn't work. Initially, I got the dreaded spinning beach ball....but eventually it worked. Then I tried playing some songs....so far so good. But then I noticed that the iCloud Status for every song showed "Error". I was still deciding whether I even cared about this since all of my songs were upgraded to 256mbps when it resolved itself and now shows "Matched". So, all is well except that since I didn't let iTunes copy the upgraded files to my library....it is still using the external drive. That's easily fixed by changing the iTunes media location to the local drive and checking "Copy files to iTunes...Library....".
Remember when I said that my situation was a bit more complicated? Well, that's just the start of it. First off, my full library of songs is actually over 46K songs....which makes me ineligible for iTunes Match. Initially, I am using iTunes match to upgrade songs <256mbps. And I am changing computers. And I decided to re-rip cd's for songs that were <256mbps which iTunes Match couldn't match. So, next I have to merge the re-rips with the iCloud upgraded songs. Once that is done, I have to import the rest of my library (songs that are already 256mbps or greater. Then most of my library will be upgraded and ...I am done right? Nope. Then I have to create a 2nd library of <25K songs that I want in iTunes Match. Guess it will all be worth it when it's done.