It would be a very simple tool to make.
The first step in reading the demo file is to split up the records by entities and time, the last step in producing a new demo file is to merge the records together into one file.
The only thing you need to be careful with are the timestamps, which have to be converted to absolute during the split and back to relative during merge.
I had something in PHP to handle the splitting, merging would be much simpler.