The migration to etcd3 blocks kubernetes upgrades, as from 1.13 etcd2 is no longer supported as a protocol. Upgrade in place for etcd is not an option really. It's a impossible process as it requires upgrading from 2.2 to 2.3, then to 3.0 and possibly to 3.1 before upgrading to 3.2 (which is what buster and stretch have).
The alternative is to setup 3 VMs and setup a new empty etcd cluster on them, then reinitialize all clusters.