Just because I'm anal about this term, "seceeding to" is not the proper terminology. Secession is the severing of ties. You don't "secede to", you "secede from". In BattleMaster parlance, secession specifically refers to a duchy leaving a realm to form a new realm. When a region (or even an entire duchy) swaps from one realm to another, it is a change of allegiance.
Yes, I know it sounds like I'm being pedantic, and I am. At least a little. But the incorrect use of this specific terminology causes a lot of confusion, especially in reporting bugs.