I spoke with Dr. Gregory Stoute (works BU admissions) last night regarding new letters of recommendations for re-applicants. He said that it will depend on your situation. If you've graduated from the universities then it will be hard to obtain new "good" letters from science professors. Unless you've already built a great relationship with them and in that case you've probably already asked them for a letter last year . In this situation it's okay to reuse your old letters. You should however obtain a new/updated letter from a dentist/advisor that could update your most recent work.
The message he wanted to get across is that dschool wants to receive letters that will allow them to assess you as the person you "currently" are.
I think the best piece of advice i can give you is to call up the schools you are planning to apply to and ask them directly.
