My answer in not 100% official, but:
No... since the original 788c don't have the 800 kHz signal built-in in the unit.
Technically, the unit should be able to use the 455 kHz signal for down imaging but I don't think they will do it to protect the sale of the new units.
Some have tested the SI transducer with a 700 series unit in Quadrabeam mode with some interesting results.. I do believe that a firmware revision could take care of this.