Prosperous The goal is always to make upgrades user friendly and seamless. That's primarily why we advocate for building extensions rather than modifying files directly, as it will make reapplying customizations easier after an upgrade. But, as you note, Flarum is still a beta product and much can change. The finalized API document for stable has several database changes that will need to be made, and this might make the upgrade process a bit rocky.
The easiest answer at this time is, we don't know. I know our core devs will do everything they can to make the stable version backwards compatible, and for now that's the best answer I can give.
EDIT: See, and I should ask Toby first on these things. ?
Toby Zerner @tobscure 04:25
yeah there will be an upgrade path from beta to stable