If you get up to level 20 prepromoted you can use a Second Seal to promote into any accessible class.
That said, I finally got my 3DS bundle today. I'm playing on Casual Hard and just finished recruiting Donnel, Gaius, and Panne. The game is a completely different beast when one only has to worry about not letting two units die instead of everybody.
I think I have 2 of those, but I read online that the best use for them is to use them when you are promoted and level 20 because you keep most of the stats and go back to level 1 again.
It depends on the circumstances. IMO your unpromoted units should wait until level 20 before promoting/reclassing as each persons' internal level counter will be at its lowest. After promotion your unit's internal level counter will be set to [20+current level] and will apply exp penalties accordingly. After reclassing your unit's internal level counter will equal the sum of the previous and current class levels. And if you should decide to class down from a promoted class your internal level counter will be cut in half (you'll effectively be half the level you used to be) but it cannot reduce your internal level counter below 20. And finally, each difficulty level has its own internal level counter cap (50 for Normal, 70 for Hard, and 90 for Lunatic).
Now to give an example of what this means lets say that we have Avatar, Frederick, Donnel, and, lets say, Cordelia, and that each of them have the same internal level counters set to 35. Assume that for Frederick, he hasn't had much use outside of Pair Up so he hasn't done anything but level 14 times to level 15 for the Great Knight's final skill Guard Boost+. And lets assume that we wanted Donnel out of Villager at level 15 since he now has Underdog and can gain nothing else of value from the class, and that you found that the Mercenary skill Armsthrift would be a great investment so you've reclassed him once. He'd have to be at level 20 as a Mercenary which is no good either so he class changed again to Hero for Sol after learning the last Mercenary skill which would leave him at level 10 Hero. Now for Avatar you wanted Spectrum Rally ASAP, so you promoted him/her at level 10 and now you have it at Grandmaster 15 which puts his/her internal level counter at 35. For Cordelia though you showed heavy favoritism throughout the game so you not only had her learn Galeforce as a Dark Flyer, but then you immediately reclassed her down to Mercenary to pick up Armsthrift and all the way through to Hero for Sol and Axe Slayer and immediately reclassed back down again to Dark Mage at which point you have to ask which difficulty level you're playing through since Normal would allow her to reach Dark Mage level 10 whereas either Hard and Lunatic would likely only allow her to reach Dark Mage level 7 to get to the same internal level counter as everyone else.
The final score from the above comes out to 14 level ups and two skills for prepromote Frederick, 23 level ups, 1 class change, and 4 skills for Avatar, 32 level ups, 2 class changes, and 5 skills for Donnel, and 40 levels (accounting for starting level), 4 class changes, and either 9 or 10 skills depending on difficulty level for Cordelia. What this means is that Frederick and Avatar have midgame level stats and skills, Donnel has endgame level stats and skills, and Cordelia has postgame level stats and skills; yet all of them will be treated identically in terms of exp gains.
Of course, none of this accounts for necessity caused by the difficulty of the situation you're facing which may necessitate you to promote or reclass someone so that you have someone who can effectively deal with whatever roadblock is blocking your road currently. Moreover the penalty for an early promotion isn't nearly as harsh in this game as it is in others.
The tl;dr of this post is that you'll miss out on some easier Exp gains for an early promotion, that the only stat losses that occur when changing levels are the stats being contributed by the class's base stats, and that you should watch to make sure you don't leave any major gaps unit coverage (i.e. don't turn all your best units into flyers on a map that hosts nothing but Snipers, Sages packing wind magic, and/or enemies packing beast/dragon slaying weapons).