The only reason they are doing this is cost. There is absolutely no return on returning a CRJ-700 for a CRJ-900. They want first class seats wherever possible. Because of this, they will take four seats out of a 70 and operate it as such.
It is a good move for DAL in the fact that they retro-fit an existing jet. What it also means is that they will not be converting the 700's for 900's. Means there is are about 32 700's/900's left until they hit the 255 ceiling.