A dsm2 radio will be able to use older dsm receivers. You can't however use the dsm2 rec with a older radio. Make sense.

I would not worry about it being phased out. Don't worry about dsm2 unless you have to have a new radio. Anyone who can tell the difference in between the speed of either is blowing smoke. Manufacturers always need to push the envelope and make things better. They also know that we as rc fanatics will probably buy the latest and greatest.