Productive life of an asparagus bed is 8-20 years, so if your plants are over 8 years old, buy new. As for transplanting, not sure when would be the best time, not a good idea to do it when the soil is cold and wet, so probably April, when you'd normally plant new plants, and only then if the weather is open, fair and dry. Will need watering extremely well, and you will not be cropping for a minimum of 2 years, even if you move the old plants.

thanks for your suggestions probally will buy new.