Level 15 is usually the best time to upgrade, when you have all the abilities from that class. The reason why you shouldn't usually wait until 20 in that case is that your level progression slows down with every level you gain, so if you don't really need the extra stats from those 5 last levels, that XP is probably better used for the advanced class or the new class if you reclass. When you reach level 20 in an advanced class, you can use a Second Seal to drop down to level 1 of the same class again without losing any stats, so you can keep raising your stats until they're maxed.