They probably don't care whether it's new letters from new people, or new updated letters from previous letter-writers, it's all the same to them. They just want the most recent "snapshot" of who you are, written by a professional.
Suggestions:
- Get new letter-writers.
- Have old letter-writers re-submit letters from latst year with updates.
- Best is to do a combination of both. Get one new, from someone who met or worked with you this year, and ask for revised and updated letter from the rest.