That depends on which Lightroom you are going to use. Lightroom Classic is the version that runs similar to what you are using. It utilizes a local catalog, and stores images locally on your hard drive. If you choose Lightroom Classic then the first time you run it Classic will find your existing catalog and will prompt you that it needs to be upgraded. You allow this to happen, and this will create a new copy of the catalog that is compatible with Lightroom Classic. It will contain all of the work that you have done, will have the same name except that it will have a 10 added to the end of it. The old copy will remain on your computer, unchanged. Lightroom Classic is installed on your computer and runs just like Lightroom 5 has been running.
If you choose the new cloud version of Lightroom that stores all of your images in the cloud then it will still be necessary for you to install Lightroom Classic long enough to create the new version of your catalog because that is needed in order to migrate the images to the cloud. Lightroom is installed on your computer, and from that point forward (after you have migrated your images to the cloud) all of your images are imported and stored in the cloud.
If you subscribe to the photography plan you have access to both versions as well as Photoshop. Which version do you plan on using primarily?
I am tagging on to your reply, because it seems to include the answer to my problem, but I want to check?
I have been also using LR5 for many years, with an extensive folder of images (about 350GO - 100K jpg images). I just subscribed to the Lightroom cloud package in the hope that my image folder could be transfered to the cloud as is. But LR seems to want to duplicate this image folder for which I don't have enough storage room on my PC.
How do I go about this transfer of my images folder to the cloud and the pick-up of my existing catalog, corrections and keywords by the new LR cloud formula?
I'm afraid I'm not the best person to ask because I primarily use Lightroom Classic. But if your intention is to move everything to Lightroom, the cloud version, then you are going to have to install the trial version of Lightroom Classic long enough to upgrade your catalog to a version that will migrate. After the migration process is complete Lightroom Classic can be removed from your computer. I have never migrated a catalog to Lightroom. The catalog will not look the same in Lightroom. Your images will not be organized in folders like they were on your computer. They will be organized by date, and you can organize them into albums. You only have a single catalog to work with. And you won't have all the features that you may have become accustomed to using in Lightroom 5. Keep watching this thread. Someone with more experience with switching to Lightroom will probably be able to give you more insight.