Hi my Italian friend,
there are 2 basic ways of doing things with styles and voices
for playing a specific song:
a)Use the the Style Creator to modify the style(change it's tempo,change it's parts etc)
While you are inside the style creator you can call the Mixing Console and change the values of volumes the effects of the style parts.
Then you can save the style as a new style(with the modified style parts).
To change the R+L hand voices of the OTS in this case you change the voices as you wish (Memory Button+OTS 1 or 2 or 3 or 4) and re-save the style.
Now your style has new modified style parts and new modified OTS.
b)You can use Registrations to save "stuff"
In regs you can save some stuff but that doesn't mean that Regs can change preset or user styles or they can change the OTS of preset or user styles.
E.g. you can take a preset style and save it in a reg with a different tempo without entering style creator at all.
The reg will use the tempo you tell it to use because you have ticked "Tempo" in the pop out window.
That doesn't mean that the original style now has a different tempo.
It means that if you cal the reg can play it in a different tempo.
The Reg cannot save OTS (There is not an option for OTS as you can see in the pop out window)
But the reg van save different R+L hand voices if you wish.
In your case:
Change what you wish for the OTS and resave the style with the song name.
Change what you wish using style creator and mixing console for the style with the song's name (e.g just the right tempo for the song) and save it again.
Now you can save the song style also to the reg buttons
Method 1:
Example:
Reg 1 it has Intro A
Reg 2 it has Main A and OTS 1 (Verse 1)
Reg 3 it has Main C and OTS 3 (Chorus 1)
Reg 4 it has Main B and OTS 2 (Verse 2)
Reg 5 it has Main D (without phrase 2 channel) and OTS 4 (Chorus 2)
Reg 6 it has Main C and some other combination for R+L hand voices (Bridge/instrumental part)
Reg 7 it has Main D and some other combination for R+L hand voices (Chorus 3)
Reg 8 it has Main D and some other combination for R+L hand voices (Chorus 4)
Before a Main comes in, you can also choose whether an auto fill in comes in or not.
While all the regs are using the same style you can use also the Style Control and OTS buttons at any time while playing.
Method 2:
Of course you can save each style in just one reg and then continue to play a song with JUST the Style Control and OTS buttons(But NOT the other Reg buttons).
Example:
Reg 1 "Felicita"
Reg 2 "A casa d'Irene"
Reg 3 "Tu vuo' fa' l'Americano"
etc
A Registration in my opinion is the solution to play along with constantly different backing tracks(style tracks) to avoid their repetitiveness if we wish.