I can't remember selling anything I ever regretted. I usually wait WAY past when I think I'll be ok without it.
Letting go for a new one, well, I know letting go of an "old reliable" with it's memories attached might need some thought, but if it is being replaced, then I would think that would help ease the pain.
I know this is not the same, but I find it funny about myself that whenever I decide to reorganize my "things" each time I let go of more and more things that made the cut so many times before. It's funny how it matters sometimes and then one time, it doesn't ! As I get older, more "things" don't matter than do.