The upgrade path for LR6 is different. Apparently you can only purchase the version six upgrade from Adobe directly (perhaps trying to force users to the Creative Cloud?) and after you make the on-line purchase you must be sure to copy the version 6.0 serial number while at the Adobe site. During install it is this serial number (version 6.0) that you need to supply when they first ask for a serial number. Subsequent to entering the version 6.0 serial number, you will receive a message confirming that this is a valid serial number but also requesting you to supply the serial number of a qualifying product (i.e. LR 1, 2, 3, 4, 5 ) which you must also enter.
The thing that was not clear to me was that during the install, when you are first requested to enter a serial number, I assumed that a qualifying (previous version) product serial number was what they wanted. It took about three hours to figure that out ( I entered each of the five previous version serial numbers and it still would not accept them). I find Adobe Systems a very difficult company to do business with. I hope this helps those who are upgrading Lightroom but not going for the Creative Cloud option. The upgrade price is $79.00 and the full price is $150.
P.S. Adobe also sends you the Lightroom V6.0 serial number via e-mail if you forget to write it down when you visit the web site.