I wonder what's the point of asking for new LOR if it's going to be weaker.
There isn't really that many students who keep in touch with professors who taught you 2~4 yrs ago. My professors all forgot about me or my name
that's your job to stay in contact with the professors.
Old letters won't hurt if you are applying to different schools the second time, but like the other poster mentioned schools such as UDM will not accept old letters. The schools that declined your application wants to see improvements in all areas if possible.