Do you want to change service, or just the boxes. If the latter, why? Is there some capability you feel you lack?
See if there is a Verizon store in you area. Discuss your plan with them. That is frequently the best plan, as you can wring out all the issues.
Unless some of the boxes are free, there is absolutely no reason not to update to HD boxes. And some improvement in service does exist. And HD channels can be viewed although in SD, but generally improved picture quality.