Alright, how exactly does class change work, especially with respect to skills?
It looks like normally you get skills at levels 1, 10, 5, and 15. If I class change a level 15 Grandmaster to Dark Flier, what happens? Am I now a level 15 Dark Flier with all the Tactician, Grandmaster, Pegasus Knight and Dark Flier skills? And what are my stats? The same ones I'd accumulated with my Tactician/Grandmaster levels?
When changing a unit's class to another with either a Master Seal or a Second Seal, their stats change and they go back to Level 1. Stats that go over the new class's stat caps (this would only happen with the use of a Second Seal) are reduced accordingly but are remembered internally within the game. Therefore, you will eventually cap all stats if you continue reclassing.
Using a Second Seal to change a Level 15 Grandmaster to a Dark Flier reverts that unit to a Level 1 Dark Flier, retaining all Skills that unit has already learned. While only five Skills may be set as active at one time, no Skill is ever forgotten by a unit regardless of class changing.