Your user styles and registrations will work fine. I am in the process of helping a friend with this exact transition.
HOWEVER, we have found that there are often better styles in the SX900, so you might want to consider going through each registration and experimenting with some of the new styles to see if there is improvement.
We often make the mistake of trying to make our new arranger sound exactly like our old
one.
Just one thing among many that I have recently discovered: Limit file names to 12 or fewer characters. We tried to load CDG files and they were great, except some of them failed to show up. I contacted Yamaha and the tech suggested checking the length of the file names, and he was 100 percent correct. I'm not certain if this applies to ALL file names or just the CDG files.
Bear in mind if you do use CDG files, and shorten the name of the MP3s, you will also have to change the name of the associated lyrics file (CDG) to be exactly the same and stay in the same folder.
Probably not many will use CDGs but this might help those that do.
Still, having all those Regs to start with will definitely make the transition easier.