Yes, you can as Lightroom 6 and CC2015 are same versions.
Lightroom 6 is the standalone version which does not include features like mobile syncing, dehaze wherease CC2015 is the subscription based version which includes these addtional features.
Yes but it can be a little tricky.
Both LR 6 and the CC 2015 version are exactly the same. The difference is how you sign in.
Sign In with an Adobe ID that has is or has been associated with the subscription plan and it becomes the CC 2015 version.
Sign In with an Adobe ID that has never been associated with the subscription plan and it becomes the 6 version. So since you already used an email address to activate the stand alone 6 version you should use a different email address, a different Adobe ID, to buy the CC version. So if you ever want to go back to the 6 version you Sign Out with the email address, Adobe ID, that IS associated with the subscription and Sign Back IN with the email address the IS NOT associated with the subscription.
Thank you! I think this is what I needed to know.