Android checks for incompatibilities between system version and app minimal required version.
Obviously, to make these congruent you can change either one.
The thing is, this will only have any utility if the app maker overestimated the level of API that they are using.
If an app is using Jellybean introduced API features no amount of lying is going to get that app to work.
So if you still want to play with lying about versions, do yourself a favor and modify the app, not the system.