from the faq:
Why doesn't the Creative Cloud version of Lightroom take the serial number from my previously purchased version of Lightroom?
You can’t use the serial number from previously purchased versions of Lightroom when you install the Creative Cloud version of Lightroom. Creative Cloud products license with your Adobe ID. Even though a Lightroom Creative Cloud installation includes a serial number, it's only used for registration, not installation.
Can I still purchase Lightroom outside Creative Cloud?
Yes. Lightroom will continue to be available as a stand-alone product, available for purchase with a traditional perpetual license.
Thanks for the answer. Yes I'd seen this but it doesn't seem to cover my situation. I don't want to use a licence key from a 'previously purchased version of LR' - I'd never installed LR prior to starting the trial. I wish to purchase it now.
i think it does, but you can test yourself by checking if you can enter a serial number to license your current trial (help>activate).
if you don't see help>activate, uninstall your cc trial, purchase and install and activate with your serial number.
Thanks. Seems complicated and contradicts the help at Installing and Activating a Try & Buy product
When I uninstall LR won't I lose info and settings which I'll have to set up again when I install the standalone version?
The programs are actually different in their licensing mechanism, so you have to uninstall the CC version of LR and install the non-CC version—your work should still be there when you install the other version. The fastest thing is to download and install the latest update to LR 5 which is LR 5.6 from the Adobe Updates page:
If you want to purchase LR 5 standalone (non-CC) then click on the Products link at the very bottom of any adobe.com page. Be sure to buy the full version not the upgrade version if this is your first purchase of LR. If you already have LR 5.6 installed when you purchase LR, you don’t need to run the installer, again, just enter in the serial number you are sent/shown.