I'm thinking something like this may very well be in the works, as well. I don't know if they'll use the "SX" prefix up and down the line, but I think there are some changes coming to the E-series. For one thing, this year, we were all expecting the replacements for the PSR-E263 and E363, which would've naturally been the PSR-E273 and E373, but that did not happen -- at least, not yet -- and it's late July. Then they came out with that weird PSR-E360 variant, which is essentially a current PSR-E263 with a touch sensitive keyboard added that is available in different colors.
Yamaha used to use a designation of simply "PSR-xxx" up and down the line, as there were entry level keyboards like the PSR-100 or PSR-200, to more advanced models like the PSR-500 and PSR-600, and then the dedicated pro models like the PSR-1000 and PSR-2000. They may just do the same thing again, and have everything from a PSR-SX200 to a PSR-SX900, but because the "S" is still in there, I'm thinking that the "SX" designation will still just be for the 600-and-up series.
For the E-series, I predict one of two scenarios. Either Yamaha will introduce a PSR-EX series, as in PSR-EX300 and PSR-EX400. Or, maybe the PSR-E360 really is the new model of the PSR-E263, instead of it being called the PSR-E273, and maybe they'll do the same with the E363 and E463 and add 97 to the model number for the next version to come up with PSR-E460 and PSR-E560 for the next versions of those models.
EDIT:
While checking out my local Craigslist, I just saw something that might very well explain what is going on with Yamaha's numbering system -- at least with the E-series keyboards. It turns out that there has already been a Yamaha PSR-273 -- not E273, but just 273. Still, that is close enough that I bet Yamaha didn't want to cause confusion by introducing a model called a PSR-E273. Thus, the E360? We'll see how other E-series models play out!