That actually depends on the motherboard. Some AM3 chipset boards did offer support for AM3+ CPUs after the fact, just needed a BIOS update. The sockets are physically the same as AM3+ boards are backwards compatible with AM3 CPUs, so the chip would fit, but whether or not the motherboard would be able to use it is another story.
You're better off running an AM3+ chip in a motherboard actually designed to use it, though.