Sorry for the misunderstanding.
I presume that:
if I would be in "_Prod" and "_HR" my "Prod$" would be "U:" and my "HR$" would be "V:".
If you would be in "_HR", your "HR$" would be "U:"
If you told me you stored something on "U:" for me, I would have to search for it. You could send me p.e. an incorrect link via e-mail.
Maybe one day, you would also need to be in "_Prod". At once, after the change, al your mappings shift one down.
Anyway, all of this is quite confusing to users and should IMHO be avoided. That's all.
Maybe I am wrong about the definition of "musical drive letters".