In general, systemd-xdg-autostart-generator is the tool that converts .desktop files to systemd user services. This allows the desktop environment to use their existing mechanism to configure autostart, but actually use systemd user instance to manage the services.
The mechanism implemented by /etc/xdg/autostart is generic, and systemd-xdg-autostart-generator tries to be DE-agnostic. AFAIK, it has been adopted by GNOME. I don’t know the status for KDE. This kind of thing is best done in cooperation with the developers of the DE, because it’s very likely that adjustments need to be made in how the DE starts processes. Thus, I’d recommend continuing the thread on discuss.kde.org rather than here.
Actually, what Gnome has done is converting all autostart units to real user units stored in /usr/lib/systemd/user.
For other DEs, you could get problems because the programs could already be started by the session manager. The systemd-xdg-autostart-generator can be useful for DEs which don’t have their own session manager.
So to converts, you should create systemd unit files in /etc/systemd/user, and at the same time disable the /etc/xdg/autostart units.
Afaik the benefit of this (very limited) approach is to be init system agnostic.
But it should be pretty trivial to convert these entries, and yes it should absolutely be done with devs. But I will test this first with noncritical processes and see if it works well.
@vekruse I am currently writing a script that does this on an existing system. When it is ready I can share it and more people can test it in a VM.