As Drake to, the real trick is converting the rest of the MIDI stuff enabled by SFF2.
Yamaha sometimes use the name "SFF GE (Guitar Edition)" instead of SFF2. (Technically, "SFF2" refers to the MIDI marker at the beginning of an SFF2 style file.) In short, SFF GE improves the realism of MegaVoice guitar parts.
Then there are non-SFF2 related things to be converted for an older keyboard: replacing new DSP effects with old ones, replacing new voices with old ones, etc.
All of this is pretty labor-intensive. If you're doing your own styles, it's sometimes easier to start out with SFF1 and convert to SFF2! At least the SFF1 style will play on a newer keyboard (as you mentioned) and you can convert at your convenience.
All the best -- pj