Accessing your iTunes library from other computers and iOS devices is the chief benefit of iTunes Match, but it's not the only benefit. You can use iTunes Match to upgrade the low-bit-rate tracks in your library to 256kbps AAC files.
After subscribing to iTunes Match and letting it back up your library, all you need to do is create a Smart Playlist to round up all the songs in your library with bit rates less than 256kbps, take the leap of faith of deleting those files, and then use iTunes Match to download 256kbps versions of the songs you just deleted. Here are the steps required to perform this audio upgrade:
Step 1: Create a Smart Playlist
Go to File > New Smart Playlist. In the first line of the playlist, choose: Bit rate is less than 256kbps.
Hit the plus sign on the right to add another parameter, and then choose: Media kind is music.
Next, create a condition by clicking on the plus sign while pressing Option. You will create two conditions. First, choose iCloud status is matched. Then hit the plus sign and for the second condition, choose iCloud status is purchased.
Click OK and from the left panel, give your new playlist a name.
Step 2: Delete all files in your new playlist
Take a deep breath, highlight all of the files in your new playlist, hit Option-Delete to delete all the files, and then choose Move to Trash. Better yet, back up your iTunes library and then hit Option-Delete and Move to Trash. Make sure you leave the box unchecked (unchecked, underline "un") next to "Also delete these songs from iCloud." (You want to leave copies in Apple's cloud, which you'll download in the next step.)
If you want to run a quick test before taking this leap of faith, just highlight a couple tracks or an album's worth of tracks and try this step with a smaller group of files first.
Don't worry, your files return instantly. Or copies of your files stored in the cloud via iTunes Match, that is. Right after iTunes deletes your files, you'll see 256kbps copies in their place, and each file will have the little cloud icon next to it, showing it's stored in the cloud and ready for download.
Step 3: Download 256kbps copies of your files
From your playlist, highlight all of the new 256kbps copies of your files, right click, and choose Download. If your playlist updates itself before you take this step, removing the songs because they no longer meet its criteria, just go to your music library and click on the cloud icon from the top menu bar to sort by songs stored in the cloud. You may need to double click on the cloud icon to get the cloud-based copies at the top. Then just go down the list and highlight all the songs in your library with the cloud icon, highlight them, and then right-click to download them.
Better yet, create another Smart Playlist with these three criteria:
Location is iCloud
Location is not on this computer
Media kind is music
Then hit Command-A to highlight all songs in the playlist and right-click to download all of them.
If you have thousands of songs to download, you may want to do this before bed, letting iTunes run overnight. In my case, about 5,800 songs from my 6,800-song library had bit rates below 256kbps.