1) I have a song specific style on a memory stick with OTS instrumentation not to my liking.
2) The goal is to set the instruments, levels, etc for all 4 variations and keep it permanently for that song.
-ok, you wish to change OTS voices in styles.
-Do I want to modify the style OTS and bring internally or leave it on the memory stick?
-it doesn't matter. OTS parameters will be changed in style that you select (wherever that style is).
Let's say you have 20 styles on USB, which you don't plan to use soon (you only wish to change OTS parameters). In this case it makes no sense to copy them to keyboard, change OTS and copy back to USB -just do the work from USB. Later you can copy modified style to keyboard anyway.
For what you wish to do (change OTS voices, so they suit better for particular styles), I don't recommend involving registrations at this point. OTS is style setting and it should be stored in style. If it happens, that particular song (for which style will be used in future) needs more settings than can be stored in style, then you should consider using registrations.
I noticed that many are using registrations just because they are "at hand" and probably end up with a mess, by not knowing what they have in registrations.
The way I see it, registrations are song specific. We pick a style which serves as basis and make additional settings (i.e. keyboard split point, pedal usage, assign buttons, L+RH voices, tempo, etc.), so it suits playing particular song and save that as a registration. Now we have our song ready (without style being changed).
Bogdan