It depends on what your versions of LR and PS are, now.
LR 6 or LR CC 2015 will upgrade a copy LR 5 and prior catalogs before use.
If you already have LR 6 then LR CC 2015 will use the catalog w/o a conversion.
What will happen when you upgrade is that your existing catalogue, if it is LR5 or earlier, will get copied and upgraded to the newest version and will be used with LR CC/ If it is a LR 6 catalogue then it will just use it directly. If it's a pre LR6 catalogue it will still exist on your hard drive in the same place as it is at the moment, but it will never get updated with any changes made in CC: it will effectively remain dormant. You would still be able to open it with your current version of LR, should you wish to keep your current version of LR on your computer, although I can't see any reason for ever using the old version if you have LR CC...
Be aware that you will not be able to open the new catalogue with your old version of LR however.