I played a bit bztogl from GNOME and forked it under https://invent.kde.org/carlschwan/bztogl.

It works quite well and preserves the creation time and author a task and comment (after applying a small patch to gitlab). The script import all the issues from a phab project and move it to a gitlab repository.

The biggest problem is to handle tasks that are shared between projects. Where should we move these tasks?

An idea would be for a task that is for example between VDG and Plasma to move it to the Plasma team and ping the VDG with @team/vdg, so that both get notified.

Maybe we could also start moving unproblematic boards first for example KMyMoney who doesn't have any shared tasks and then work on more difficult boards like Plasma, VDG, Promo, Documentation, ...