That looks like a very odd request indeed. Quite funnily, on a day where we were mentioning the issue of Mathcad Prime not preventing us from redefining/undefining pre-defined variables and functions...
I would certainly never recommend to undefine or redefine pre-defined variable, and units even less (although I did take some liberties with this rule, regarding imperial units, but apart from Americans, British and crazy people, who chooses to use those anyway ?). Using units everywhere where possible is the first line of defense against errors and typos.
Like any rule, it can be broken when one knows what he is doing, but from my little experience, it causes lots of problems more often than not...