If you have any letters that are older than 2 years, I would get them updated. Even if it's asking the letter writer to just slap on a new date.
If you have a significant volunteer/research/work/academic activity that really stands out on your application but you didn't obtain letters for previously, then I would try to remedy that. Nothing raises a red flag like claiming you volunteered for several years in a nursing home or played 4 years of college-level sports, but then not having a letter to show for it. Also, if you strengthened the weaknesses in your application in any way (getting more clinical experience or research or whatever), then I would try to secure a letter from that as well.
Other than that, make sure your letters are all strong. Don't reuse any in which you might question the contents.