Wow - that brings back memories. When I was framing in Los Angeles, I was a Lucid Lines dealer in the early 70's and I am very familiar with the frame you are discussing.
Lucid lines had two types of frames; a single piece of metal that was notched in three corners and folded around the "art" and joined at the fourth by a pressed piece of "L" metal, or 4 pieces of mitered moulding that were all joined at the corners using a pneumatic press that pushed the stamped "one way" steel fasteners in the corners. They were NEVER intended to come apart
Does yours have the glass art in the front channel and a piece of cardboard in the back so the art is "floating" off the back, or is yours an "open back" and you can see right through the art to the wall?
We did take them apart many times when the frame got damaged or the client wanted to "upgrade". The stamped corners do not go all the way up to the channel where the glass is. We used a cut off wheel on a Dremel tool to cut through the stamped corners- being careful not to get the frame/glass too hot. You only have to do this on two corners, then you can slide the glass out.
If yours has the cardboard backing, pull it out (you can slice through it being careful not to scratch the glass on the other side - but there is an air gap). When it is removed, you can get a better view of the stampings holding the corners together.
Please be sure you and your customers understands that it is not without risk - but it can be done.