Iljitsch,
Jari basically said this ^^However, although I agree it's true in the general case, I think for non-trivial servers/services, this isn't true: if there is a good reason to make changes on the IPv4-only side, it may be possible to make them.
To clarify, I think it should be "the IPv4 stack in an IPv4 only host is unmodifiable". I would would think that small modifications to the IPv4 side of a dual stack host is possible and have likely happened already. This would be inline with your observation about dual stack servers.
Bob