Oops -- I meant for example the PSR S500, which also only has the USB midi port. You're right -- the YPG625 and E403 are pretty similar feature-wise, so not really much a point using the YPG625 as a controller when it's pretty much got the same capabilities as the board it's controlling.

I've got the YPG625 currently, and plan on eventually getting a more powerful and portable (61-key) board for the road (for example, maybe the S500). But when at home I would like to be able to use the 88 weighted keys of the YPG625 with the more powerful features of the portable unit. If that future keyboard only has USB midi, will I be able to interface the YPG625?