I'll probably end up waiting until one of the popular instances migrates to 0.19, then I'll quickly work on migrating to 0.19 and release a new version. The main alexandrite.app site will use the 0.19 version once lemmy.world updates, because that's its default instance. Exact specifics might change depending on how the rollout goes and how much free time I have around then, so this might change. If Alexandrite updates before your server updates you might just have to use the default UI until your instance updates.
For self-hosters who want to wait on 0.19, Alexandrite's releases have version numbers nowdays so instead of using the latest docker tag, I'd recommend using the last non-0.19 version until you upgrade Lemmy.