Some cards are basically just cut down versions of other cards, in this case the 6850 and 6870 are both the same card except the 6850 BIOS only uses some of the hardware available while the 6870 BIOS uses all of it. So when people figured this out, they flash their cards to higher versions.
Its not a guaranteed thing through, the 6950 and 6970 were the same, and after a while AMD just started cutting off the cores physically to prevent people flashing their 6950's to jump a performance tier without paying. Its also possible that your 6850 is a defective 6870, but they just disabled the defective shaders/cores or whatever and sold it as a working 6850.
Go ahead and try it I think. If it works you have a free upgrade, if not then you haven't exactly lost anything.