class Foo
end

Foo.object_id                   # => 34581480

f = Foo.new                     # => #<Foo:0x000000041f53c0>

f.class.object_id               # => 34581480

# This just prevents the "const redefined" warning
Object.send(:remove_const, :Foo)

# "Reload" class Foo
class Foo
end

Foo.object_id                   # => 34580180
f.class.object_id               # => 34581480

f.is_a?(Foo)                    # => false
f.class == Foo                  # => false
f.class.name == "Foo"           # => true