Well, we don't know how many miles OP has. I suggested above that it would take a decade to transfer half a million miles that way. I think a situation where it might make sense is that OP is moving to a non-UA hub, and has a small number of miles. For example, if OP had 50,000 miles, s/he may not have enough for a nice trip on UA, but could be willing to take the hit to increase his or her Skymiles to an award level. That way the whole 50K would be wiped clean and a Delta trip would be in the offing.