This is a watered down version of what I've heard.
From an athletics standpoint, the biggest concern for coaches that I'm hearing is mostly due to the differing start dates, larger break times, and how seasons may end up being extended or overlapping. Obviously a lot of sports overlap, but it's still possible for multi-sport athletes to compete in several due to the limited time period of conflicting days/weeks. There are also rules in place by the UIL that limit the contact hours with students during extended breaks that would have to be addressed. The summer strength and conditioning programs and specialized sports camps to help the athletes improve would also be impacted.
In the long run, if they switch to extended calendars everything will adjust. The biggest concern is the upcoming school year and how that will affect the kids in any type of contest based activities (athletics, band, choir, debate, FFA,etc... )
Everything right now is based on the traditional school calendar, so it's cause for concern on how it will play out if that is changed.