From your size changes, it sounds like the rest of your body got smaller and your breasts just stayed the same. I wouldn't worry too much about it right now. People lose weight from different areas at different times. For me personally, I can usually see the change first in my chest, and my legs hang on to everything they can. You still have 60% of your weight to lose, there is a very good chance that you will see lose from your chest at some point along the way.
For the insurance question it would definitely depend on your coverage. If it was causing you back issues it very well might. But I don't think it is something to panic about yet.
This bra size equivalency explanation might be a bit of a comfort. According to this, 36G and 42DD are equivalent. You just now have a smaller rib cage.
http://www.sizechart.com/brasize/sistersize/index.html