1. we just had a nice topic about it http://www.gamedev.net/topic/677700-planet-rendering-spherical-level-of-detail-in-less-than-100-lines-of-c/
2. you rather update the VBO with what is visible. You'd not fit all LODs in one go into vmem.
3. I've told one solution in the topic from 1. the thread started implemented it, I think the source is public, check it out, it's easy.