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.