Did they agree to take the guest blogging stint to promote their careers / work generally (or just for fun)? Or did they do it for the purpose of increasing the page rank of whatever websites they happen to have an interest in?
If it was one of the former reasons, I don't think whatever changes are coming down the pike should be a big deal. Although I can't speak for google, I imagine that whatever algorithm they use will look for pervasive patterns. So assuming that this guest blogging stint is a rare or unique thing for these guys whatever changes are made will probably have no effect at all (i.e. the dofollow links will still "work"). The next most likely thing to happen is that the algorithm somehow figures out that they are guest posting and disregard the any backlinks. Finally, there's a possibility that there will be a slightly negative effect. If it does turn out to be the case there is a penalty, then in the future you can configure the CMS software to put nofollow on guest post links and then it will certainly be neutral.
Regardless, they will still all get the exposure that comes from getting their names out in front of your blog's regular audience.