Hmmm, let me check a Cosmic Down....OK. It is an OK bag with a rather high temp rating at 32F. It weighs about 2#9. (Note these all very a bit.) 550 fill is not that great hence the cheaper price tag. Durability is still great, well worth taking care of.
OK. If you cannot afford to replace that bag, then don't fiddle with it. Quilting is great in warmer months, but quickly looses it's appeal in late fall/early spring. 3 major points here: ventilation, ground protection, weight.
Ventilation is great in the summer, but the same drafts that are cooling in summer are cold in winter. There is a lot of air leakage compared to a bag.
GOOD ground protection is required. The conduction of heat is the strongest source of heat loss. With a quilt, there is none. With a bag you loose the loft, but you retain a 1/8"-1/2" of insulation against the ground. This isn't a lot but it helps. A simple Blue Pad, will cover the difference for a 32F bag. However, a 32F quilt, will not. You need a good R2.5+ pad for good comfort at 32F. The pad becomes part of the sleep system and is really necessary. A holed inflatable is a real disaster with a quilt. 'Corse, in the forest, it is easy enough to scrape up forest duff to 6-8" for some insulation.
Weight is great. Quilts are generally smaller than an unrolled bag. So, you carry less weight. Don't forget a pad, but mostly, you will carry a pad, anyway. It may be a better pad, or two pads...negating any weight savings, though.
I would leave your current bag. If you want a quilt, save for one. In the mean time, use the bag as a quilt when you get a chance. If you get cold, zip it up. If you still get cold, well, you are at the limit of your current equipment. Add a layer or two. Don't forget that the EN ratings assume a good pad and long johns inside a bag. You may find the survival rating of 20F to be quite cold. I believe most would have rated it as a 35F or more bag.