No an existing 401(k) plan can not be amended to a safe harbor 401(k) plan as soon as the plan year begins so January 1, 2015 would have been too late to amend the plan to safe harbor.
And existing 401(k) plan would have had to hand out the safe harbor notice for 2015 by December 1, 2014 and amended the plan to safe harbor effective 1/1/15 by December 31, 2014 to be safe harbor for 2015.
If the Plan was a profit sharing ONLY plan that was adding a NEW 401(k) feature, October 1, 2015 would have been the deadline to give the 3 month deferral window for new plans.
See the 401(k)(13) regs for additional info.