I was originally going to apply during the 2012 application cycle and prepared my letters of recommendation accordingly but plans changed and I instead applied during the 2013 cycle. I did add two letters but included the "old" letters from 2012 as well. I had absolutely no issue whatsoever with my application materials this past cycle.
You'll be fine as long as you add a more recent letter (as you intend to), more to show you've been recently active in a productive activity, not because your older letters are worth any less. Good luck!