Extensions on multiple entry tourist visas

I have a specific situation. I have a multiple entry tourist (L) visa and my 90 day stay will expire 2 days before my friends and I can leave for Malaysia to renew it. I have been told that I can get an extension fairly easily, but my real question is whether or not I can reenter China. Will the extension be treated as a separate visa in and of itself, or can I continue to use my multiple entry tourist visa after getting an extension. Will I need to apply for a brand new visa when I try to come back into China?