How bad is using a letter that’s a year old

I am having to push my medical school application back and already got letters from my professors. The thought of asking one of my professors again to update their letter gives me physical pain. Is it ok if 3 of my letters have updated dates and 3 are old? The letter I really don’t want to update is one from a lab I did 2000 hours of research in and have a pub in review.