changing plan period will it affect current members?

We had a plan that was effective this calendar year and next. Anyone who signed up prior to Jan. 1, 2016 had the whole year plus all of 2016 and their membership will expire on 1/1/17.

We need to change it to an annual plan, so if someone signs up on Jan. 5, 2016 their membership will run 1/5/16 - 1/5/17.

I can change the plan no problem, but want to make sure that it won't alter any of the original member expirations of 1/1/17. Can you let me know if that will work? Otherwise I can defunct the old plan and start a new one I guess but it would be easier to change it on 1/1/16.

It shouldn't as that's all calculated (start and expiry dates) and stored at time of subscription. However, it is not recommended to alter a plan with existing subscribers, but rather to disable new subscriptions to the plan and simply create a new one. Please be sure to test carefully and make a backup if you decide to change a plans duration.