If you already own LR 6 and have a valid serial number for it you should sign up for the Photographer subscription plan using an Different Adobe ID, a different email address, and have the use of both. LR 6 installed on 2 computers and the CC PS & LR on 2 other computers. All other restrictions applying.
If you really want to do the subscription plan I suggest you use a different Adobe ID to sign up. Other wise if and when you ever cancel your subscription you can always go back to LR 6. If you use the same Adobe ID that ID will always be tied to the subscription even after you cancel, not renew, and make it harder to activate LR 6 again (you would then need a second Adobe ID to continue to use LR 6).
Oh and yes there is a restriction of signing out and back in. IIRC it is a Time Limited system. If you sign out on one system and sign in on another system you have to wait X amount of time before you can do that again. I think it is something like 48 hours but I'm not sure. Check the Adobe KB for more details, it is covered someplace.
Thank you:-) - I must admit I hadn't thought of creating a new ID. I've found a discussion here that basically says there shouldn't be problems with a reasonable number of activations (2nd post from bottom) - but I'll hang on to my standalone LR6 as well (at least until LR7...)