Slowly working backwards unwork your stitches one at a time by sticking the right needle into the completed stitch on the left needle then slowly and gently pull the yarn out of the st. Then on to the next st. Make sure your sts are leaning the correct direction. I find this the easiest way to unravel a long amount of sts.
If you ask there might be someone here that has enough yarn to complete the whole BO row plus a little extra. You could unravel the BO and part of the last row. Add the yarn , finish the last row with the new amount added and then work the BO again or another BO that uses less yarn. This way where the yarn is added shouldn't show. I hope I explained that clearly.