I am certain FST, whenever it happens, will replace All Program Draw for that two week cycle. It will not happen in conjunction, especially with the draw size having gone up. I don't know if or when it'll happen, but it is almost a guarantee that it will replace the regular draw during that week.
For me, it will never happen that way, because of two reasons :
1. Doing just a FST draw, lets say on the 31st, they will have to issue probably 2,500 ITAs to them. They never allocated more than 1,000 ITA for FST throught the years.
2. They will never leave out a PNP draw, no matter what. So, if they conduct a draw solely for the FSTs, then it will be PNP+FST, still close to 1,800 ITA for them if they do it on the 31st of August. More than 2,000 if they do it on September the 14th.
Highly improbable on both cases in my opinion.