So, practically speaking, if the Beacon Chain is to launch on Mainnet for 2020Q1, and we follow the 6 month cadence described in this thread, the issuance change on ETH1 would have to either be as part of a single-EIP hard fork or as part of an April 2020 one (meaning Q2).
This means the EIP would have to be ready for November 2019, at which point details about the beacon chain’s launch may be more fleshed out.