Indeed, you are seeing efforts by some sleeping-bag makers to come up with "multi-temperature" bags that can be modified as conditions dictate. An example: Sierra Designs' Mojave ($159; www.sierradesigns.com), a 15-degree Polarguard-filled bag that can be morphed into a zero-degree bag, or even a minus-15 bag, by adding different bag "lids" (the lids are available in Polarguard or down and range in price from $69 to $139). The lids in turn are attachable to a ground cover to make a super-light summer bag rated to 40 degrees or so.
These systems work, but to be honest I'm not terribly enamored of them. I think they're exceedingly complicated, and you end up spending about as much as you might on two straight-rated bags for different temperatures. I mean, once you "buy into" a setup like this, you're committed to buying a bunch more products from the same company. And while Sierra Designs makes excellent stuff, I'd still rather be able to shop around.
So for someone in your position, I'd be inclined to drop for a 30-degree bag. That will be comfortable enough in warmer temps, but packs enough insulation to keep you healthy when the weather cools (I'll assume you're in a tent, which is warmer than sleeping in the open). Even a 20-degree bag won't cost too much for that temperature range. And there are a zillion good choices. Moonstone's 3D Stratus, for instance, is rated to 32 degrees, has synthetic fill, and costs just $150 (www.moonstone.com). Marmot's Fusion 30 combines down and synthetic insulation to take advantage of the warm-when-wet advantages of synthetic fabric and the comfort and compressibility of down. Rated to 30 degrees, this costs $169 (www.marmot.com). Best of all may be Integral Designs' Andromeda Strain, which will keep you warm well below its suggested 40-degree rating, has soft Primaloft insulation, and costs $170 (www.integraldesigns.com).