There are a few confusing responses here, so let's clarify: if you are going to let iTunes Match expire, make sure you have downloaded the music that is in the cloud to your local hard drive first. Once it is there and you turn off iTunes Match, those items will be left untouched on your computer. You will no longer has access to what was in the cloud.
One of the great benefits to iTunes Match is that you have the opportunity to have all of your music converted to the higher bitrate of 256 kbps. Whatever is matched in the cloud is available to re-download at this bitrate. It does not automatically do this to your library and only matched songs that iTunes Match has from the iTunes Store are available at this bitrate.
I have found it is easiest to go into Album view in iTunes (select an album you know is in a lower bitrate) and delete it from your local hard drive. The cloud icon will appear below that album cover now. Re-download at the higher bitrate by clicking on that icon. There are a couple of prompts when you delete locally, do not check the box that asks you if you also want to delete it from the cloud for obvious reasons.