I think all of the 83XX series and 8600 through either 8604 or 8605 are getting retrofitted with the Heart seats. A lot of them are done already (I flew on 8307 and 8600).

As the poster above said, this is being done so those Evolve seats from the -800s can be used to complete the remaining -700 acquisitions.

Most of them will be Evolve Blue before going on the -700s.