It is not possible for existing subscriptions to be exchanged or converted to Nature+, however you are able to purchase a Nature+ subscription on top of any existing personal nature subscriptions.
Annual nature journal subscriptions are subject specific and, along with new monthly issues, provide access to the entire back catalogue of journal content from 1997 for Nature and longer for some other journals.
Nature+ allows access to 50+ nature family journals, but only includes content from 2017 onwards.