I emailed the MarcEdit listserv for advice and now have a regular expression that I can modify that will hopefully be able to take care of a large chunk of the "smooshed" fields in our older records. Very, very nice, plus I plan on taking this regular expression apart as part of a project to learn more about MarcEdit's regular expressions. I've used regular expressions in Microsoft Word (they're called wildcards there) and have done some nifty things, so I'm excited about the prospect of learning how to do similar things in MarcEdit.
Now I just have to carve out the time for all of that "playing." All the webinars last week and the time I spent figuring out how to globally change the type on our thesis and dissertation records lost me most of the small lead I managed to gain over the Processing students, so it may be a while before I'll have the time to do a lot.