Ethanol doesn't have as much power stored per gallon as Gasoline, so you need to add more ethanol to the air to get correct mix.
Flex fuel injectors are "larger" so they can run E85 in a comfortable duty cycle, not maxed out.
You could switch any carb engine over to ethanol you just needed to put in larger jets, for the same reason as above.
That's why the injectors are different.
Can you interchange them, the injectors, yes, but............you may get a Lean or Rich code after a while, engine is never running Lean or Rich, thats not what those codes mean.
The computer for a flex fuel engine is programmed for the larger injectors, it "knows" how much fuel should flow in when it opens that size injector for XX milliseconds at 65psi fuel pressure.
If you use smaller injectors then it would have to open the injectors longer to get the same amount of fuel.
After a time computer would "think" something may be wrong, and it will notify driver with CEL and code.
Same thing when using larger injectors on a gas only computer