For completeness, checking out this other answer you can redefine methods of global functions if you use the fully qualified name, i.e. prepending the name of the module:
julia> f = 1
1
julia> f(x) = x
ERROR: cannot define function f; it already has a value
Stacktrace:
[1] top-level scope
@ REPL[233]:1
julia> Main.f(x) = x
# no prob!