To be precise, there is a single installer image that you boot and use to create the partition layout. Then you pick a stage 3 tarball and unpack it into the root partition, and this tarball is either OpenRC-specific or systemd-specific.
Yes it is possible to change your mind and switch from one to the other after installation, but it's not straightforward.
Yes it is possible to change your mind and switch from one to the other after installation, but it's not straightforward.