In general watermelon plants do not transplant well. If you would like to try to transplant them you should dig your new hold twice as big as the plant you are transplanting. Make sure to water the hole before you begin. It is best to do the transplant near sundown as the plant will be under less stress at that time. Take special care not to disturb ethe roots, this is the biggest challenge you will have. Make sure to keep all of the roots intact with the soil. You should also add a good soil mix with good drainage. Once the transplant is complete, water the plant again and a bit more often until the plant is back to good form. They may wilt the first few days, if this happens, continue to water. They should survive.