Quick instruction on the class change seals and when to use them?
I got one or two and my chars are getting past level 10, so wanna make sure I don't make mistakes.
I'm going to make this one last detailed post about using seals. Then I'll just link to it from now on.
Starting with the
summary/tl;dr version:
Master Seals promote a unit in a base class (tactician, lord, cavalier, etc) into an advanced/promoted class (grandmaster, great lord, great knight, etc). In general it is best to use a Master Seal either 1) when a unit hits level 20 (not 10) in their base class, or 2) after the unit has started to hit their stat caps (green stat numbers) in their base class.
Second Seals are new in FE Awakening, and change a character to another class. You can change to any base class available to that character, and as a promoted unit you can reclass into other promoted classes after level 10. As a level 20 unit, you can reclass back to the same class you were in to keep gaining experience.
In general, it is better to reclass with Second Seals as soon as possible to enter the class you want to get to. So for example, if you want your avatar to go to mercenary for Armsthrift, it's most effective to do it as a level 10 tactician.
Now the "why" part:
For Master Seals, when you promote a character the game automatically assumes you were level 20. When you go from Lord -> Great Lord, even if at level 10, the game will treat the unit as level 1 (displayed level) + 20 (base class levels) unit. This affects your experience gain, so if you promote at level 10, your unit will be a level ~11 unit statistically, but gain experience at the rate of a level 21 unit. This means those levels were effectively skipped, and less frequent levels relative to your stats will make the game more difficult.
Second Seals are more complicated. In terms of when you should use them they are basically the opposite of a Master Seal - if possible putting off a Master Seal as long as possible will lead to better
stat growths for your character. On the other hand, using a Second Seal sooner rather than later will benefit your experience growth in the long run and help you collect skills faster.
To keep reclassing balanced the game keeps track of all your levels in previous classes through an "internal level." Each time you use a Second Seal, the game adds levels to this internal level, which combined with your displayed level (+20 as a promoted unit) determines your experience growth. The exact formula for the internal level is:
(displayed level [+20 if promoted] - 1) / 2, rounded down.
This means your levels early on in the game will be the fastest and easiest levels to get. If you are concerned about stats, you can take advantage of this and reclass frequently early on. Not only is this useful for gaining both stats and base class skills, but you will actually level faster this way than just getting to level 20.
Say you reclass at level 10, that adds 4 internal levels and you become level 1, so you gain experience at the rate of a level 5 character. But statistically you are level 10. You get to level 10 in this second class and reclass again. This adds 4 more levels to your internal level (so 8 levels), and you are level 1 again, so you will gain experience as a level 9 unit despite being statistically level 20.
This isn't really as good for skills for promoted classes since the minimum you are adding to your internal level is 10 with each reclass. While promoted units can reclass at any time, most of their best skills come at level 15 meaning you are looking at 17 internal levels. Still, you should reclass ASAP rather than holding off until 20 - Level 5 if you just want the level 5 skill and go back to a base class, level 10 if you want to go to another promoted class immediately, and level 15 if you want that skill.
It's also worth noting that each difficulty level has a cap on the internal level. For normal mode it's 20, for hard it's 30, and for lunatic it's 50.