After spending a lot of time digging through various resources, I've come to the conclusion that the LP-7 does not appear to be an option on the 625. If I had a YPG-625 to look at, I'm willing to bet that it lacks the specialized PEDAL UNIT accessory jack required by the LP-7.
Comparing the two keyboards, the 635 is the newer of the two units and they are very similar.
Best I can tell, one of the most compelling apparent differences between the 625 and 635 is that the 625 doesn't appear to have any option for the three-pedal LP-7 attachment. The YPG-635 manual available on Yamaha's web site shows a procedure and a specialized connector (separate from the sustain jack) for adding the LP-7. The 625's manual does not describe such a procedure or show such a "PEDAL UNIT" connection in teh specifications under "auxillary jacks" nor does the 625 manual list the LP-7 as an optional accessory.
The other strengths of the 635 appear to be 64 note polyphony vs 32 note, 3 more voices, 20 more reverb types, 20 more chorus types, DSP effects.
Overall, piano players are likely to want the 635 so they can get their hands on the inexpensive LP-7 3-pedal assembly (if it ever goes off of the backorder it's on now in dec 08).
