Is this lathe or mill that you are looking at? Which OneCNC demo are you looking at, Xp series or 2003 series?
AFAIK, there would be no ready method of doing what you require.
So would you be calling up these speed changes with M functions? Can you shift on the go?
I also have a similar problem as you, with a lathe using a 16 speed headstock, controlled by M function. The post configuration is not as flexible in the 2003 series yet, as it is in XP, but Lathe XP has not been released yet.
I'll give you a chance to answer so we can narrow down the field a little bit.