What happens when due to the buying/selling of stock, companies are no longer a controlled group part way through the year?

[katieinny]

A group of companies with 401(k) plans was probably a controlled group. We were trying to get ownership percentages so we could get them on the right track, but in the meantime some stock was bought or sold, and as a result they are no longer a controlled group. Now I'm trying to figure out how to treat them for 2010. It doesn't make sense to go through hoops to make them comply with the controlled group rules for 6 months, or whenever the transaction took place. What happens when controlled group status goes away part way through the year?