Ability to matriculate typically is contingent on actually completing your current degree program, so even if you're in a masters degree you typically will have to finish that even though a masters is not explicitly "required" for acceptance. A UG degree is explicitly required so there's no way you'll be allowed to start school without finishing that first. Some schools even put expectations on your performance in your remaining classes--meaning you can't just tank all your classes and barely pass to get your degree.