I use the following:
osc vc --file=$package.changes.proposed $package

where $package is in this case qemu or qemu-linux-user (see qemu package
sources if you want context, in particular the update_git.sh script. In this
specific instance, I working on this IBS project/package:
https://build.suse.de/package/show/home:bfrogers:branches:Devel:Virt:SLE-15-SP1/qemu).

The file passed has content I want automatically added to qemu.changes or
qemu-linux-user.changes.

This command no longer works as before, instead I am left with a new file:
$package.changes.proposed.changes, which is wrong.