One more (much technically challenging) solution would be to assign EVM version and gas prices to contracts at deployment. That means, that smart contract that is deployed before the hard fork is always executed with old gas prices (and old features of EVM).
So, we will have EVM0 (pre Constantinople) and EVM1 (Constantinople). When a new contract (running EVM1) calls anything that is deployed before that, EVM1 communicates to EVM0, and the old contract will use old gas prices and old assumptions will stay the same. This communication isn’t trivial, but since contracts have very specific interfaces it is not impossible.
Cons:
- more complicated codebase and testing;
- more complicated contract interaction;
- bloating codebase with any hardforks;
Pros:
- contracts that are already deployed aways will stay the same and behave the same;
- incentive for those who can to upgrade their contracts to the new version because cheaper gas, etc.
I still think that this might solve the whole class of problems like that and might be worth it in the long run because the contracts behaviour would be truly immutable.