So I have an alchemist. She's level 7. I want her to eventually become a ninja and continue up the alchemy tree. What's a good level to change her over? I want her to get access to extremely high spells first.

Change once your leveling is slowing down; once you class change, you will level up very quickly at first..

Due to the way the game works, the higher the level at which you change, the worse your physical attack accuracy will be in the long run. Yes, it's stupid, but that's the way the game works.

You should always level switch immediately after a level-up so you don't waste any xp, because switching resets you to 0 xp, none of your progress carries over. Switching at level 8 is normally a great time. It takes as much xp to get from level 8 in a given class to level 9 as it does to get from level 1 to level 8. Ninjas have higher xp requirements than alchemists, but if you do it then you'll be at most 1-2 levels behind the rest of your party and you'll catch up very quickly.

Because alchemists and ninjas are both alchemical casters, the only threshold you need to reach to cast high-level alchemy spells is the skill requirement. If you've been putting most or all of your academia points into alchemy you should be fairly close. (Level 6 spells at 90, level 7 spells at 98.) Ninjas get fewer academia points per level than alchemists, but you'll get betting 8 full levels instead of one, so you still should come out ahead.