There are a couple schools of thought out there on this. One is that search engines consider individual pages and not full websites, the other is that in the eyes of the search engines individual web pages are part of the full website. In other words, is there really domain authority or not?
According to the first group, domains only appear to have authority because of the linking structure of most sites. Because the pages on a domain are generally linked together quite extensively, much of the individual page authority is passed between the pages and thus it looks like the domain has authority, but in reality, Google doesn’t care if the pages are on the same domain, it’s just giving various amounts of authority to individual pages. For example, the homepage is usually the strongest page on a site not because it’s the homepage of a domain, but rather because most of the time the homepage is linked to by every sub-page and the homepage usually has the most external links pointing at it. So, it’s a strong page. If this group is correct (and I’d lean in this direction), then redirecting the homepage of XYZ Corp shouldn’t affect the microsites since the homepage doesn’t link to them and they don’t link to the homepage.
According to the other group, not only do pages obtain authority, but the domain they are a part of also has authority and that domain authority is shared to some extent with every page on the site, regardless of the internal linking structure. If this group is correct, then redirecting the homepage would affect the microsites.
Sorry to leave it a little ambiguous, but I haven’t seen an experiment which dealt with this specific situation, so I can only speak in theory. Personally, I’d lean towards saying that it shouldn’t have an affect on the microsites, but I can’t say that for sure.
If you do redirect the homepage, please keep us informed as to what happens. It would be great to learn for sure what kind of impact something like this would have.