IANAL, but if the added GPL3 code is, as Richard said, an "isolated
utility" that is not linked to any of the GPL2+ code (an assumption about
the nature of an "isolated utility"), then isn't this "mere
aggregation"?
If so, wouldn't it have no effect on the licensing of the GPL2+ code, and
the resulting package simply contains a mix of GPL2+ and GPL3 components?
I'm not arguing that there's anything wrong with relicensing the GPL2+
source to GPL3, only that I don't think it automatically occurs in the
described scenario. If it was desired to relicense the GPL2+ source to
GPL3, I think that should be done explicitly, by actually changing the
license notices.
And it's not what Richard was asking, but I think if the isolated utility
generated documentation based on the GPL2+ sources, the resulting
documentation would be GPL2+, unless the isolated utility included some of
its own GPL3 components in that output.
Eric