The decision on whether or not to replace the existing containment cap on that is capturing oil from the Gulf spill hasn't been made by BP, or government officials, who are weighing the positives and negatives of whether or not to go ahead with the task.
If the cap were to be replaced, it would capture more oil on a daily basis, but also could temporarily release more oil into the Gulf.
It seems what is being decided is whether or not to wait for the first relief well attempt to plug the oil leak, which is ahead of schedule at this time.
Why this actually makes sense, is the efforts during the storm season have already resulted in stoppages, and a window of good weather would be needed to result in a successful changing of caps.
Assuming its success, it would make mute the rest of the strategies to capture more oil, as it would permanently plug the oil well.