The entire catalog and all images & videos would be imported. I'm not sure in that instance whether LR would recognize duplicates already in the LR catalog. So you could end up doubling the space taken on your hard drive. It might be better to add recent folders one at a time. Or risk giving it a try and return to a LR backup catalog if necessary.
I'm pretty sure that upgrading the PSE 14 catalog will create another Lightroom catalog (in addition to the ones you may have already), and you'd have to decide how to merge the Lightroom catalogs.