I've been thinking about the question in the title for a while. What I want to achieve is something like this (sorry for the crappy image):
Since the digits are lit, it cannot be just a simple texture change - either the digits themselves (all the 7+7 ones) are changed or there's a dark texture without lit digits, and another "layer" in front of it, with transparent textures of course. Another way is to model all the numbers (or all the digits separately) and change those from script, but that would be a nightmare to implement, also I don't think it would be very optimized (from the point of LOTUS).
I think the number model with transparent but lit textures would be the most optimized method, though this way it would use 10+1 textures for both numbers (10 numbers with zero and one empty for shut down speedo).