Depends upon the model of transmission. On some, it is no big deal, on others, the fluid will foam and possibly shoot out of the filler tube. Others, it will cause the transmission to slip and possible overheat.
You're going to have to drain it out. Most tranny's don't have drain plugs; you'll have to loosen the tranny pan bolts and drop the pan 1/2 inch or so, and let fluid drain into a BIG pan. Use a clean pan, so you can put correct amount of fluid back in, and not have to buy more fluid. It's messy, but no additional cost, unless you can't get tranny pan to seal up again, in which case you'll need a new gasket, which is cheap.