That notice is advising you that there is a paid upgrade available. That doesn't mean that you must take the upgrade. If you notice on that screen there is a checkbox to indicate not to show it again. If you choose that option Lightroom 5 will continue functioning as it has in the past. If you decide you want to upgrade the new version will create a new copy of your catalog. All of your work would be intact, and your work would continue normally. But the choice is yours. If you're satisfied with Lightroom 5.7 just check the box so it doesn't show again and life will go on. Incidentally, that notice caught a lot of users off guard when it first appeared. It is the general consensus among users that it was a rather poor method of introducing a new version.
I will read through the upgrade improvements and decide, since I don’t have great deal of time invested in Lightroom to date.
I downloaded the update and now it is indicating that I can "Begin TRIAL"!!!
this is what I was afraid of - that it is changing me to a subscription with a new version and disregarding that I had a paid version. Is this true?? Luckily I kept the old version when I downloaded the update. Can I delete only the new install??