When you upgraded from LR5 to LR 6/2015 your old LR5 catalog was migrated to a new LR6 format catalog.
LR 6 cannot use the LR 5 catalog, nor LR 5 use the LR 6 catalog.
Your LR 5 catalog will remain unchanged while you are using LR 6/2015. However, if you decide to go back to LR 5, there is no way to get the edits from LR 6 back into LR 5. The best you could hope for would be to save your LR 6 edits in the XMP files and import the new photos into the LR 5 catalog.
That said, the easier thing to do if you wanted to continue using LR 6/2015 after stopping your CC subscription would be to purchase the standalone version of LR 6, which uses the same catalog as LR 2015 subscription.