Reclassing isn't too complicated.
For pre-premotes, use Second Seals to learn whatever skills you want to learn before promoting. Good options are Vantage from Myrmidon, Armsthrift from Mercenary, Veteran from Tactician, Locktouch and Movement + 1 from Thief, and Miracle and Healtouch from Cleric (only for units who will use staffs obviously). Anything else is up to you, but I think the rest are easily replaceable by better skills from promoted classes.
Important factor: For Second Seals, use them at Level 10. I know that goes against all logic with Fire Emblem, but trust me. When you reset to level 1, you keep most of the stats you had from Level 10, thus giving you MORE stats. Why Level 10?
[hide]a. Most units gain their second skill at level 10. There are units that gain their second skill at level 15 (dancer, manakete, tactician, villager, taguel), but most of those skills aren't that useful Special Dance is useless outside of Dancer, Wyrmsbane is useless outside of fighting, Solidarity is kind of meh, Underdog isn't always useful. Beastbane, taguel's skill, is the only one I'd consider potentially useful.
b. Most importantly: When you use a Second Seal, your unit's internal level is changed. Say you were Level 14 when you used a Second Seal to become a Mercenary? Your level is put through an equation: (Your Level - 1) / 2 = (Answer rounded down)
So…
-(14 - 1) / 2 = 6.5
-6.5 is rounded down to 6.
When you get your new unit, your unit will be level 1, but will have an internal level of 6. What this means is, although your unit is level 1, he/she will gain the same amount of exp as if he/she were level 6. So that when you change classes, you don't level up as fast as you normally would. So ideally, you want to make sure your level is the lowest it can possibly be while still learning the skills you want to learn.
Level 10 is the lowest level you change classes with, so ((10-1) / 2 = 4.5). Your internal level will be level 4, which is far more manageable.
Take note that this isn't required, but it does make your life so much easier as it won't take as long to level your character. The game does however have unlimited EXP, technically so it's not the end of the world if your character is level 20 and using a second seal.[/hide]
Once your done learning all the pre-promote skills you want to learn, go ahead and promote at level 10-20. If you're going to change promoted classes later (ie, Hero -> Myrmidon), then don't bother going to level 20. If you're going to promote to your next class and stay at that class, then go ahead a level all the way to 20 and then promote.
After you've promoted? You learn skills at Level 5 and Level 15.
You can still use Second Seals too!
-At level 1, you can use a second seal to return to pre-promote classes (although, I wouldn't suggest unless you feel it's completely necessary).
-At level 10, you can use a second seal to change into ANY PROMOTE CLASS YOUR UNIT CAN BECOME!
For example:
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ie. Olivia's classes are Dancer (no promotes), Myrmidon (Swordmaster and Assassin), and Pegasus Knight (Dark Flier and Falcon Knight). So if Olivia promotes to Assassin…
Use Second Seal at Level 1 - Dancer, Pegasus Knight, Myrmidon
Use Second Seal at Level 10 - Dancer, Pegasus Knight, Myrmidon, Dark Flier, Falcon Knight, Sword Master, (Assassin, but she's already in that class).
Also, the same effect works with special classes. So if Olivia is a Dancer...
Use Second Seal at Level 10 - Pegasus Knight, Myrmidon
Use Second Seal at Level 30 - Pegasus Knight, Myrmidon, Dark Flier, Falcon Knight, Sword Master, Assassin
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Hope that helped a little bit. Same internal level rule applies for promotes.
Side note on turning promotes to pre-promotes (ie. Fredrick, Libra, Say'ri, etc). By all means, turn them into pre-promotes. I'd recommend learning the skills of their current class before doing so though (ie, making sure Fredrick learns Luna and Dual Guard+). Yes, pre-promotes can use the skills of promoted characters.
In some cases, I would recommend turning a promote into a pre-promote.
Like, Fredrick can become a Cavalier, Knight, and Wyvern Rider. He already knows the Cavalier skills, and the Knight and Wyvern Rider skills suck. So I'd recommend keeping him promoted.
But with Say'ri, she become a Myrmidon, which has Vantage, which is always good.