Well the quick way is to copy your Lr6 previews folder to a new location, beware that the size of this can be pretty big, it will depend on a number of factors, mostly the number of images. I don't know if 6 got the update that sized preview images to match the screen width, I'm on CC for the HiDPI support, with a 5K display, so my standard previews are now 5120×3144 px, larger than the 100% views. With 50K images my previews folder is 80 GB! With your existing previews safely backed up you can now let the system update itself, and during that process the previews folder will simply have an -2 appended to the folder name. this will allow LrCCC to use the existing previews, which will be replaced when you make any changes to the image, as per normal.
It's probably best to do it this way, as you don't really want to have to have Lr regenerate you standard previews, as they will all now by default be "screen width". Lr CCC will want to rebuild the whole lot in the background by the way. If you go back to Lr 6.13 then after you remove the CCC program you will simply need to delete the previews folder tree, along with the new catalogue, also with a -2 suffix appended. Until I renamed my catalogue folder, and previews tree, I had got to -2-2-2 on my catalogue name. Because of the size of the catalogue folders and two full sets of previews it pays to check that you will have enough disk space to start with. i had my catalogue on my main system SSD, only a 233 GB drive, and in the update process I managed to fill the drive . If that happens the installer will leave behind some lock files, and you won't be able to either start the old version or restart the conversion of the catalogue file, as the lock files will stop both processes from starting. The only option then is to delete all the extra files in the folder with your .lrcat file manually.