Problem: no easy way to upgrade contract bytecode

[yihuang]

We can take advantage of the builtin gov process to do contract upgrade, implement a contract upgrade proposal type, proposer submit the contract address and bytecode, after proposal passed, the contract is upgraded.

Advantages

• contract address don't change
• don't need any dev effort in contract level

Disadvantage

• need the whole chain gov process to upgrade an individual contract, so probably only works for important contracts.