You should be able to just turn Time Machine on. Keep in mind that when you do turn Time Machine on again, Tine Machine will probably think that everything needs to be backed up again. So your previous backups should be fine.I do have other pre-Sierra backups. For me the question is rather if I want to keep the ability to roll files back to different states (e.g. 1 month ago, 5 months ago, and so on) or if it is cleaner to start over.