Upgrading the catalog to Lightroom 6 will not change or modify your catalog in any way, and Elements will continue to be able to use it. The programs do not share the same catalog. Any work you do in the Elements catalog after the upgrade is not known by Lightroom, the two catalogs do not speak to one another.
My advice is for you to try Lightroom on new photos that are not in Elements, see if you are comfortable with Lightroom on these new photos rather than doing actual work on photos that are in BOTH Lightroom and Elements, as work ddone in Lightroom does not automatically transfer back to Elements.
Please read the free e-book for beginning with Lightroom: http://www.lightroomqueen.com/
Thanks. I do most of my editing in Photoshop, but use elements for the catalog feature, so I already have to deal with pushing changes back into the catalog if desired. I now have the full Creative Cloud, and figured I'd give Lightroom a try. I'm not sure I'm ready to replace Photoshop (I do significant image creation, not just photo enhancing) but am intrigued by it. I'm also concerned about the life span of the Elements organizer, now that similar file handling is in other Adobe products, and as I have so much invested in the tagging and such that I have done, I want to see how it might move into the "products of the future". It is a relief to know that I can "upgrade" without losing what I have done. I'll give it a try. Thanks again.