It depends on the program - I believe that Chicago doesn't require to you maintain employment. So if you stop working you can start taking daytime classes or a bigger course load (but you can't officially transfer to the full-time program). You can take quarters off anytime (you just have to finish within 5 years), so you can take off for an internship if you want.
At Kellogg, on the other hand, they require you to maintain full-time managerial level employment throughout your MBA (their definition of managerial level is pretty flexible though). If you lose your job, then you have to notify the program and you have a quarter or two to find a new position. They'll help as they can - they just need to know you are making a legitimate effort to find a new job. You are not allowed to take more than the standard 2 classes without specific permission. And students can transfer to the full-time program, but only at a certain point in the program so it doesn't really have anything to do with students losing their jobs.