"Thus if the PSR only is capable of 4 bar loops and they can't be modified to accept the longer loops, I think it is useless to convert Korg to Yamaha. Now, Yamaha converted to Korg may be a different story. I really haven’t a clue about that aspect."

The newer PSR can be set to any number of bars for style parts.Unfortunately,Yamaha makes very little use of this and it seems to be implemented mostly on Jazz or Big Band styles.I think part of it is to keep the style file to a smaller size to allow room for more onboard styles.
I have converted Korg to Yamaha but the results were usually less than satisfactory...usually an issue with note limits and chord recognition rather than the sounds.I have used parts from Korg styles with much greater success in making new styles using PSR Style Creator.They have lots of movement and provide a nice addition to a Yamaha style.

Ian