Whonix 14 users who started with Whonix 14 KDE can still use it until Whonix 14 gets deprecated (which is 1 month after release of Whonix 15) which is hopefully shortly after Debian 10 / buster release.
Whonix 14 → Whonix 15 will most likely have instructions how to migrate/upgrade from KDE to XFCE (unless this is technically too difficult to figure out but expected quite easy).