I'd have to understand what your notion of identity is, and how that identity can be transferred around.
If I say x: y does add 'x 1 then change y as well? It doesn't seem it would if you didn't a-priori have an allocation of a bignum (or at the very least, force an allocation on that assignment and sync it in both variables).
If it doesn't transfer, then you have the situation where your in-place mechanics are confined to a single variable name in a single scope, and becomes a new allocation when it leaves the scope.
It's good to consider degrees of freedom like this, and how the language parts can be used.
But an infix addition operator that modifies its left argument seems like an unwanted axis. And taking a WORD! to ADD seems to me stranger than just being willing to say that all ADDs mutate.
(If you want to use + as prefix, you would write + -< b c ... see SHOVE)