LR CC 2015 and LR 6 are the exact same program. Same download and same files get installed on your system. So YES LR 6 will work with a catalog created in LR CC 2015.
The difference comes from the Adobe ID you use to Sign in with.
You do not have to uninstall LR CC and install LR 6. Look at this link.
That is to serialize the trial of LR CC but should work for going from CC to 6.