If you are going from Lr6 to LrCC, you don't need to download and reinstall it. However, doing so will allow you to install the latest update and makes it easier to ensure it is authorised with your CC account.
Don't be misled by the comment "A conversion from old to new cannot be reversed.". If you are going from an earlier version of Lr, remember that Lr6/CC will need to upgrade the catalogue. But it first saves a copy of the earlier version of the catalogue. This precaution lets you go back.