Some time ago I read Vitalik Buterin's blog post about possible attitude to inflation and transaction fees. From it it followed that nothing was decided yet and there are various possibilities. Has a decision been taken and will block reward and/or transaction fee mechanism change with Serenity?
Currently, there isn't anything set in stone, as the development community is debating the issue. There are arguments for reducing the issuance rate of Ether even leading up to POS. See : https://github.com/ethereum/EIPs/issues/186 . But these are still at discussion stage and not concrete code.