I wrote Walks of Italy inquiring about the Underground Colosseum Tour since I have reservations in September and there have been reports of some cancellations and some success. Here is the response.
"The Colosseum administration has reduced the tickets for the VIP Colosseum Underground tour, but we still have the possibility to offer this tour to clients that have already booked it. Unfortunately, we do not have information for the tour on September 18th, but it is confirmed now. If we have any changes to it, we will inform you in advance."
Basically, if you have already booked this tour, there is a chance you may be able to take it, but you may not know for sure until the last moment.