It is always quality over quantity, for reapplicants too.
If the experience with the letter-writer was deep, positive, and meaningful to your application and the person knows you well, over a period of years, then a one year old letter is better than a letter from someone who has only known you for a few months.
That said, I am not thrilled reading a Bio 101 letter from freshman year if the person is now taking a gap year or two.
You also do need to consider that one or more of your original letters was not strong enough to help you gain admission, and at this point, it is a guessing game for you to re-evaluate which one(s) that might be.
My advice is to get some new letters from anyone who will know you well enough by the time you need the letter to write a good letter. ASK the person first if they feel they know you well enough to write a medical school LOR. Add any new ones to your letters if you only had a few. If you already had a good number, replace it with your oldest/weakest experience(s).