Rather than buying a new bag, you could consider getting a summer-weight down quilt and just layer that over your existing bag. As a bonus, then you have just the quilt for some activities; can be comfy to use even at home. I keep mine in my emergency "if an earthquake or something bad happens" kit as a light and low volume "better than nothing" sleeping bag, but can pull it out for warm summer trips or to layer with my 20 degree bag for winter trips.

Just have to make sure the light quilt is sized large enough that it can connect and stay in place over your existing sleeping bag in a way that allows the latter to still fully loft.
Brian Lewis