At some point in the near future, the party are going to be facing a half-dragon enemy that at half health can transform into a full dragon (with full health).
The half-dragon has no healing abilities, so it won't be able to come back up above half health. Conditions won't carry over, concentration spells, such as Witch Bolt, will carry over. The party don't have any instant-death spells, and if they somehow manage to cleave through enough hit points to kill it before it transforms, it will then transform on death regardless. The dragon will have full HP when it appears.
The party consists of 5 Level 7s and 1 Level 5. The half-dragon is CR 5 and the dragon is CR 13 (it's an Adult White Dragon).
How do I work out the encounter XP and difficulty? Do I calculate it as two separate encounters, or is there some calculation I can do as a single encounter?
I'm not sure, but I think I've seen a question similar to this before, but I couldn't find it, so if you do see it, feel free to close this and link to it!